Subaru Check Engine Codes giving you trouble? The check engine light can be a source of anxiety for any car owner. At CAR-TOOL.EDU.VN, we help you understand what these codes mean, potential causes, and how to address them. Knowing about OBD-II codes, engine diagnostics, and car maintenance can give you more control when your Subaru needs attention.
Contents
- 1. What Are Common Subaru Check Engine Codes?
- 1.1. What Does Code P0420 Mean for My Subaru?
- 1.2. What Does Code P0171 Mean for My Subaru?
- 1.3. What Does Code P0300 Mean for My Subaru?
- 1.4. What Does Code P0442 Mean for My Subaru?
- 1.5. What Does Code P0128 Mean for My Subaru?
- 2. How Can I Diagnose Subaru Check Engine Codes?
- 2.1. What Is an OBD-II Scanner and How Do I Use It?
- 2.2. What Diagnostic Tools Do I Need?
- 2.3. What Are the Basic Troubleshooting Steps?
- 3. What Are Subaru-Specific Diagnostic Procedures?
- 3.1. How Do I Access Subaru Technical Resources?
- 3.2. What Are Common Subaru-Specific Codes?
- 3.3. What Are Subaru-Specific Diagnostic Tools?
- 4. What Are the Most Common Problems Triggering Subaru Check Engine Codes?
- 4.1. How Do I Address Oxygen Sensor Issues?
- 4.2. How Do I Fix Vacuum Leaks?
1. What Are Common Subaru Check Engine Codes?
Common Subaru check engine codes indicate a variety of potential issues. Here’s a breakdown of frequent codes and their common triggers:
| Code | Common Problems That Trigger This Code |
|---|---|
| P0010 | Powertrain Control Module (PCM) failure, Variable valve timing actuator failure, Wiring issue |
| P0011 | Camshaft variable timing solenoid failure, Engine oil level is too low, The engine is not timed correctly, The engine oil does not meet the manufacturer’s requirements, Variable valve timing actuator failure, Worn timing chain |
| P0012 | Camshaft variable timing solenoid failure, Engine oil level is too low, The engine is not timed correctly, The engine oil does not meet the manufacturer’s requirements, Variable valve timing actuator failure, Worn timing chain |
| P0013 | Powertrain Control Module (PCM) failure, Variable valve timing actuator failure, Wiring issue |
| P0014 | Camshaft variable timing solenoid failure, Engine oil level is too low, The engine is not timed correctly, The engine oil does not meet the manufacturer’s requirements, Variable valve timing actuator failure, Worn timing chain |
| P0101 | Large vacuum leaks, Split Intake Air Boot or PCV Hose, Defective intake manifold gaskets, Mass Airflow Sensor (MAF), Mass Air Flow Sensor circuit and or wiring problems, Defective Barometric Pressure Sensor, Dirty or contaminated Mass Air Flow Sensing wire or filament, PCM software needs to be updated |
| P0102 | The Mass Airflow Sensor (MAF) Sensor is unplugged or the wiring is damaged, Loose or corroded electrical terminals in the MAF Sensor circuit, Faulty MAF Sensor |
| P0113 | Defective Intake Air Temperature Sensor, Dirty air filter, Defective Mass Air Flow Sensor, Faulty or corroded Intake Air Temperature Sensor wiring or connections |
| P0128 | Defective Engine Thermostat, Defective Engine Coolant Temperature Sensor, Defective Intake Air Temperature Sensor, Defective Cooling System, Low Engine Coolant, Dirty Engine Coolant causing incorrect Coolant Temperature Sensor readings, Defective/always running Engine Cooling fan(s) |
| P0135 | Defective Oxygen Sensor/Air Fuel Ratio Sensor, defective Oxygen Sensor/Air Fuel Ratio Sensor Heater Circuit, Exhaust System Leak, Intake Air System leak, Low Fuel Pressure, Defective Engine Coolant Temperature Sensor, Defective sensor wiring and/or circuit problem, PCM software needs to be updated, Defective PCM |
| P0171 | Control module software needs to be updated, Vacuum leaks (intake manifold gaskets, vacuum hoses, PCV hoses, etc.), Mass air flow sensor, Plugged fuel filter or weak fuel pump, Plugged or dirty fuel injectors |
| P0174 | PCM software needs to be updated, Vacuum leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses, etc.), Faulty Mass Airflow (MAF) Sensor, Plugged Fuel Filter or weak Fuel Pump, Plugged or dirty Fuel Injectors |
| P0200 | Defective Fuel Injector, Faulty or corroded Fuel Injector, wiring, and/or connections, Plugged Fuel Injector, Dirt in the Fuel Injector |
| P0201 | Defective Fuel Injector, Faulty or corroded Fuel Injector, wiring, and/or connections, Plugged Fuel Injector, Dirt in the Fuel Injector |
| P0202 | Malfunction of the Fuel Injector, Malfunction of the PCM Fuel Injector driver circuit, Defective wiring/connections in the Fuel Injector wiring harness, Defective Fuel Injector power circuit(s) |
| P0203 | Fuel injector failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P0205 | Fuel injector failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P0300 | Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and/or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems) |
| P0301 | Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and/or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems) |
| P0302 | Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and/or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems) |
| P0303 | Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and/or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems) |
| P0304 | Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable), Incorrect ignition timing, Vacuum leak(s), Low or weak fuel pressure, Improperly functioning EGR system, Defective Mass Air Flow Sensor, Defective Crankshaft and/or Camshaft Sensor, Defective Throttle Position Sensor, Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems) |
| P0401 | Restriction in the EGR passages, usually caused by carbon buildup, The EGR Valve is defective, Lack of proper vacuum or electrical signal to the EGR valve, Malfunctioning EGR Vacuum supply solenoid, Lack of proper EGR system feedback to the computer from the: Manifold Absolute Pressure Sensor (MAP), Differential EGR Pressure Feedback Sensor (DPFE), EGR Valve Position Sensor (EVP), EGR Temperature Sensor |
| P0420 | Inefficient Catalytic Converter(s), Defective Front or Rear Oxygen Sensor(s), Misfiring engines |
| P0430 | Defective Catalytic Converter, Internal engine damage resulting in high oil consumption and/or a leaking Head Gasket has damaged the Catalytic Converter |
| P0440 | Missing fuel cap, Defective or damaged fuel cap, Distorted or damaged Fuel Tank Filler Neck, Torn or punctured Evaporative system hose(s), Defective Fuel Tank Sending Unit gasket or seal, Split or damaged Carbon Canister, Defective Evaporative Vent Valve and/or Evaporative Purge Valve, Defective or damaged fuel tank |
| P0441 | Missing fuel cap, Stuck open or closed purge valve, Defective or damaged fuel cap, Distorted, damaged or cracked Fuel Tank Filler Neck, Torn or punctured Evaporative system hose(s), Defective Fuel Tank Sending Unit gasket or seal, Split or damaged Carbon Canister, Defective Evaporative Vent Valve, Defective or damaged fuel tank |
| P0442 | Defective or damaged fuel cap, Distorted or damaged Fuel Tank Filler Neck, Small tear or puncture in the Evaporative system hose(s) and/or Carbon Canister, Defective Fuel Tank Sending Unit gasket or seal, Small split in a seam of the Carbon Canister, Defective Evaporative Vent Valve and/or Evaporative Purge Valve, Defective or damaged Fuel Tank, Defective Fuel Tank Pressure Sensor, Defective Leak Detection Pump, Slightly loose and/or worn clamps or hardened O-rings anywhere in the EVAP system |
| P0455 | Missing fuel cap, Defective or damaged fuel cap, Distorted or damaged Fuel Tank Filler Neck, Torn or punctured Evaporative system hose(s), Defective Fuel Tank Sending Unit gasket or seal, Split or damaged Carbon Canister, Defective Evaporative Vent Valve and/or Evaporative Purge Valve, Defective or damaged fuel tank, Defective Fuel Tank Pressure Sensor |
| P0500 | Defective Vehicle Speed Sensor, Defective Speedometer, Vehicle Speed Sensor wiring or connector, Communication (CAN) bus problems, Defective transmission or differential Vehicle Speed Sensor drive gear |
| P0501 | Defective Vehicle Speed Sensor, Defective Speedometer, Vehicle Speed Sensor wiring or connector, Communication (CAN) bus problems, Defective transmission or differential Vehicle Speed Sensor drive gear |
| P0505 | Defective Idle Air Control Motor, Intake Manifold Vacuum leaks, Carbon buildup in the Throttle Body air passages |
| P0506 | Defective/sticking Idle Air Control Motor, Intake Manifold Vacuum leaks, Carbon buildup in the Throttle Body air passages, Defective Power Steering Pressure Switch |
| P0507 | Defective/sticking Idle Air Control Motor, Intake Manifold Vacuum leaks, Carbon buildup in the Throttle Body air passages, Defective Coolant Temperature Sensor, Defective Charging System/Alternator, Defective Power Steering Pressure Switch |
| P0600 | Defective PCM (Power Train Control Module), Defective PCM data bus wiring/connections, Defective PCM data bus ground circuit(s), Defective PCM or other control module controlled output devices, Defective CAN bus communication |
| P0601 | Lack of proper voltage to the PCM, Defective PCM memory module, Defective PCM ground circuit(s), Defective PCM controlled output devices |
| P0602 | Powertrain Control Module (PCM) failure, Wiring issue |
| P0603 | Lack of proper voltage to the Keep Alive Memory connection on the PCM, Defective PCM Keep Alive Memory Module (KAM), Defective PCM ground circuit(s), Defective PCM controlled output devices |
| P0605 | Lack of proper voltage and/or ground to the PCM, Defective PCM ROM memory module, Defective PCM ground circuit(s), Defective PCM controlled output devices |
| P0700 | Defective Shift Solenoids, Defective Engine Coolant Temperature Sensor, Defective Valve Body, Dirty transmission fluid that restricts the hydraulic passages |
| P0705 | Defective Transmission Range Sensor (PRNDL input), Defective Transmission Range Sensor (PRNDL input) wiring or connector, Defective Valve Body, Defective manual shift valve linkage, Dirty transmission fluid that restricts the hydraulic passages |
| P0706 | Defective Transmission Range Sensor (PRNDL input), Defective Transmission Range Sensor (PRNDL input) wiring or connector, Defective Valve Body, Defective manual shift valve linkage, Dirty transmission fluid that restricts the hydraulic passages |
| P0720 | Defective Output Speed Sensor, Defective Shift Solenoids, Defective Engine Coolant Temperature Sensor, Defective Valve Body, Dirty transmission fluid that restricts the hydraulic passages, Defective Output Speed Sensor wiring or connector |
| P0730 | Defective Shift Solenoids, Defective Engine Coolant Temperature Sensor, Defective Valve Body, Dirty transmission fluid that restricts the hydraulic passages |
| P0841 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission fluid pressure sensor failure, Wiring issue |
| P0842 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission fluid pressure sensor failure, Wiring issue |
| P0845 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission fluid pressure sensor failure, Wiring issue |
| P0846 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission fluid pressure sensor failure, Wiring issue |
| P0847 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission fluid pressure sensor failure, Wiring issue |
| P0901 | Clutch master cylinder failure, Clutch solenoid failure, Transmission Control Module (TCM) failure |
| P0935 | Hydraulic power unit assembly failure, Transmission Control Module (TCM) failure, Wiring issue |
| P0942 | Hydraulic power unit assembly failure, Transmission Control Module (TCM) failure, Wiring issue |
| P0944 | Clogged transmission filter, Transmission fluid level is low, Transmission oil pump failure, Wiring issue |
| P0961 | Transmission shift solenoid failure |
| P0A08 | Inverter/converter assembly failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P0A0D | High voltage safety device failure, Inverter/converter assembly failure, Power management control module failure, Service disconnect plug is not properly connected, Wiring issue |
| P0A0F | Hybrid transaxle assembly failure, Internal engine failure, Power management control module failure |
| P0A7F | Poor connections at the HV battery, A problem with the HV battery, ECU issues |
| P0A80 | High voltage battery assembly failure |
| P0B22 | Battery contactor assembly failure, Battery Energy Control Module (BECM) failure |
| P0B24 | Battery Energy Control Module (BECM) failure, High voltage battery assembly failure, Wiring issue |
| P0B26 | Battery Energy Control Module (BECM) failure, High voltage battery assembly failure, Wiring issue |
| P0B28 | Battery Energy Control Module (BECM) failure, High voltage battery assembly failure, Wiring issue |
| P0B30 | Battery Energy Control Module (BECM) failure, High voltage battery assembly failure, Wiring issue |
| P0C00 | Drive motor generator power inverter control module failure |
| P0C09 | Drive motor generator assembly failure, Drive motor generator power inverter control module failure |
| P0C11 | Coolant system leak, Drive motor generator power inverter control module failure, Engine cooling fan failure, Water pump failure |
| P0C14 | Coolant system leak, Drive motor generator power inverter control module failure, Engine cooling fan failure, Water pump failure |
| P0C15 | Coolant system leak, Drive motor generator power inverter control module failure, Engine cooling fan failure, Water pump failure |
| P2000 | Diesel Particulate Filter (DPF) failure, Intake air leak, Mass Airflow (MAF) sensor is dirty/loss of calibration |
| PP2002 | Diesel Particulate Filter (DPF) failure, Intake air leak, Mass Airflow (MAF) sensor is dirty/loss of calibration |
| P2004 | A failed IMRC actuator, A problem with the intake manifold or linkage, Wiring issues |
| P2006 | Intake manifold runner control actuator failure, Powertrain Control Module (PCM) failure, Restricted vacuum lines |
| P2101 | Accelerator Pedal Position (APP) assembly failure, Powertrain Control Module (PCM) failure, Throttle control motor failure, Wiring issue |
| P2122 | Accelerator Pedal Position (APP) assembly failure, Powertrain Control Module (PCM) failure, Throttle control motor failure, Wiring issue |
| P2135 | Accelerator Pedal Position (APP) assembly failure, Powertrain Control Module (PCM) failure, Throttle Position Sensor (TPS) failure, Wiring issue |
| P2138 | Accelerator Pedal Position (APP) assembly failure, Powertrain Control Module (PCM) failure, Throttle Position Sensor (TPS) failure, Wiring issue |
| P2181 | Engine coolant heater failure, Engine coolant level is too low, Thermostat failure |
| P2210 | NOx sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2213 | NOx sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2237 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2238 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2251 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2302 | Ignition coil failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2303 | Ignition coil failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2305 | Ignition coil failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2308 | Ignition coil failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2310 | Ignition coil failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2401 | Evap leak detection pump failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2402 | Evap leak detection pump failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2422 | EVAP vent valve failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2431 | Air control solenoid failure, Powertrain Control Module (PCM) failure, Secondary Air System pressure sensor failure, Wiring issue |
| P2432 | Air control solenoid failure, Powertrain Control Module (PCM) failure, Secondary Air System pressure sensor failure, Wiring issue |
| P2500 | Alternator failure, Battery failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2501 | Alternator failure, Battery failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2503 | Alternator failure, Battery failure, Wiring issue |
| P2509 | Powertrain Control Module (PCM) failure, Powertrain Control Module (PCM) power relay failure, Wiring issue |
| P250C | Oil level sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2601 | Coolant Heat Storage (CHS) water pump failure, Coolant Heat Storage (CHS) water pump relay, Powertrain Control Module (PCM) failure, Wiring issue |
| P2607 | Intake air heater, Powertrain Control Module (PCM) failure, Wiring issue |
| P2609 | Intake air heater, Powertrain Control Module (PCM) failure, Wiring issue |
| P2610 | An internal PCM problem, A problem with the PCM power or ground circuit |
| P2614 | Broken tone ring, Camshaft Position Sensor (CMP) failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2706 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission shift solenoid failure, Wiring issue |
| P2711 | Internal transmission failure, Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission solenoid failure, Wiring issue |
| P2714 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission shift solenoid failure, Wiring issue |
| P2716 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission shift solenoid failure, Wiring issue |
| P2723 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission shift solenoid failure, Wiring issue |
| P2803 | Powertrain Control Module (PCM) failure, Transmission Control Module (TCM) failure, Transmission range sensor failure, Wiring issue |
| P2806 | Powertrain Control Module (PCM) failure, Transmission Control Module (TCM) failure, Transmission range sensor failure, Transmission range sensor is out of adjustment, Wiring issue |
| P2809 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission pressure control solenoid failure, Wiring issue |
| P2810 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission pressure control solenoid failure, Wiring issue |
| P2815 | Transmission Control Module (TCM) failure, Transmission fluid level is low, Transmission pressure control solenoid failure, Wiring issue |
| P2A00 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2A01 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2A03 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2A04 | Exhaust leak, Oxygen sensor failure, Powertrain Control Module (PCM) failure, Wiring issue |
| P2BA8 | Faulty NOx Sensors, NOx Sensors harness is open or shorted, NOx Sensors circuit poor electrical connection, Fualty Diesel Particulate Filter (DPF) |
| P3000 | Fuel level is too low, High voltage battery assembly failure, High voltage battery is not sufficiently charged |
| P3100 | High voltage powertrain control module failure |
| P3400 | Faulty Cylinder Deactivation System/td> |
| P3401 | Engine oil level is too low, Powertrain Control Module (PCM) failure, Variable valve timing solenoid failure, Wiring issue |
| B0081 | Wiring issues, Control module problems, A problem with the airbag |
| C0040 | A faulty wheel speed sensor, A problem with the wheel speed sensor circuit, Reluctor issues, A problem with the ABS module |
| U0001 | A faulty wheel speed sensor, A problem with the wheel speed sensor circuit, Reluctor issues, A problem with the ABS module |
| U0073 | A faulty control module, A problem with the CAN bus |
| U0100 | A faulty PCM, A problem with the control module circuit, A problem with the CAN bus |
| U0107 | A dead battery, A faulty TAC module, A problem with TAC module circuit, A problem with the CAN bus |
| U0121 | A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus |
1.1. What Does Code P0420 Mean for My Subaru?
Code P0420 indicates an issue with the catalytic converter efficiency. This means the catalytic converter isn’t cleaning exhaust gases as effectively as it should.
Several factors can trigger this code:
- Inefficient Catalytic Converter(s): The converter itself may be worn out or damaged.
- Defective Front or Rear Oxygen Sensor(s): Faulty sensors can provide incorrect readings to the engine control unit (ECU).
- Misfiring Engines: Misfires can cause excessive hydrocarbons to enter the catalytic converter, damaging it.
1.2. What Does Code P0171 Mean for My Subaru?
Code P0171 signals that the engine is running lean. A lean condition means there’s too much air and not enough fuel in the air-fuel mixture.
Possible causes include:
- Vacuum Leaks: Leaks in intake manifold gaskets, vacuum hoses, or PCV hoses can introduce extra air into the system.
- Mass Air Flow Sensor (MAF) Issues: A dirty or faulty MAF sensor may not accurately measure the air entering the engine.
- Fuel Delivery Problems: A plugged fuel filter, weak fuel pump, or dirty fuel injectors can restrict fuel flow.
- Control Module Software: Sometimes, the engine control unit (ECU) needs a software update.
1.3. What Does Code P0300 Mean for My Subaru?
Code P0300 indicates a random or multiple cylinder misfire. This means one or more cylinders aren’t firing properly.
Potential reasons for this code:
- Worn Spark Plugs: Old or damaged spark plugs can cause misfires.
- Ignition System Problems: Issues with ignition wires, coils, distributor cap, or rotor can lead to misfires.
- Vacuum Leaks: Vacuum leaks can disrupt the air-fuel mixture.
- Fuel Pressure Issues: Low or weak fuel pressure can prevent proper combustion.
- EGR System Malfunctions: A malfunctioning EGR system can affect cylinder firing.
- Sensor Problems: Defective mass airflow, crankshaft, or camshaft sensors can contribute to misfires.
- Mechanical Problems: Low compression, leaking head gaskets, or valve problems can also cause misfires.
1.4. What Does Code P0442 Mean for My Subaru?
Code P0442 means a small evaporative emission (EVAP) leak has been detected in your Subaru’s fuel vapor recovery system. The EVAP system prevents fuel vapors from escaping into the atmosphere.
Common causes for a P0442 code include:
- Defective or Damaged Fuel Cap: This is the most common cause. Ensure the fuel cap is properly tightened.
- Distorted or Damaged Fuel Tank Filler Neck: The fuel cap might not seal correctly if the filler neck is damaged.
- Small Tear or Puncture in Evaporative System Hoses: Inspect all hoses for cracks or damage.
- Defective Fuel Tank Sending Unit Gasket or Seal: This can leak fuel vapors.
- Small Split in a Seam of the Carbon Canister: The carbon canister stores fuel vapors.
- Defective Evaporative Vent Valve or Purge Valve: These valves control the flow of fuel vapors.
- Defective or Damaged Fuel Tank: A damaged tank can leak vapors.
- Defective Fuel Tank Pressure Sensor: This sensor monitors pressure in the fuel tank.
- Defective Leak Detection Pump: This pump tests the system for leaks.
- Loose or Worn Clamps or Hardened O-rings: Check all connections in the EVAP system.
1.5. What Does Code P0128 Mean for My Subaru?
Code P0128 indicates that the engine coolant temperature is below the thermostat regulating temperature. This means the engine is taking too long to warm up.
Possible reasons for this code:
- Defective Engine Thermostat: The thermostat may be stuck open, preventing the engine from reaching the correct temperature.
- Defective Engine Coolant Temperature Sensor: A faulty sensor can provide inaccurate temperature readings.
- Defective Intake Air Temperature Sensor: This sensor helps the ECU determine the correct air-fuel mixture.
- Defective Cooling System: Problems with the cooling system can affect engine temperature.
- Low Engine Coolant: Insufficient coolant can prevent the engine from warming up properly.
- Dirty Engine Coolant: Dirty coolant can cause incorrect temperature sensor readings.
- Defective/Always Running Engine Cooling Fan(s): If the cooling fans are always on, the engine may not reach operating temperature.
2. How Can I Diagnose Subaru Check Engine Codes?
Diagnosing Subaru check engine codes requires a systematic approach. Understanding OBD-II scanners, diagnostic tools, and troubleshooting steps is essential.
2.1. What Is an OBD-II Scanner and How Do I Use It?
An OBD-II (On-Board Diagnostics II) scanner is a tool that reads diagnostic trouble codes (DTCs) from your vehicle’s computer. These codes help identify potential problems.
Here’s how to use one:
- Locate the OBD-II Port: Usually found under the dashboard on the driver’s side.
- Plug in the Scanner: Connect the scanner to the OBD-II port.
- Turn on the Ignition: Turn the key to the “on” position without starting the engine.
- Read the Codes: Follow the scanner’s instructions to read the stored codes.
- Record the Codes: Write down the codes and their descriptions.
- Clear the Codes (Optional): After recording, you can clear the codes to see if they reappear.
2.2. What Diagnostic Tools Do I Need?
Besides an OBD-II scanner, consider these tools:
- Multimeter: To test electrical circuits and sensors.
- Vacuum Gauge: To check for vacuum leaks.
- Fuel Pressure Tester: To measure fuel pressure.
- Scan Tool Software: Advanced software can provide more detailed information than basic scanners.
2.3. What Are the Basic Troubleshooting Steps?
Follow these steps to troubleshoot check engine codes:
- Read the Code: Use an OBD-II scanner to retrieve the code.
- Research the Code: Understand what the code means and potential causes.
- Inspect the Obvious: Check for loose gas caps, visible hose damage, and disconnected wires.
- Test Sensors: Use a multimeter to test relevant sensors.
- Check for Vacuum Leaks: Use a vacuum gauge or listen for hissing sounds.
- Review Technical Service Bulletins (TSBs): TSBs provide information on common issues and fixes for specific vehicles.
- Consult a Professional: If unsure, consult a certified mechanic.
3. What Are Subaru-Specific Diagnostic Procedures?
Subaru-specific diagnostic procedures involve understanding Subaru’s unique systems and common issues. Accessing Subaru technical resources and understanding Subaru-specific codes can help in accurate diagnoses.
3.1. How Do I Access Subaru Technical Resources?
Accessing Subaru technical resources can provide valuable information for diagnosing and repairing your vehicle. Here are some ways to access these resources:
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Subaru Technical Information System (STIS): STIS is a subscription-based online portal that provides access to Subaru service manuals, technical service bulletins (TSBs), wiring diagrams, and other diagnostic information. This is a comprehensive resource for professional technicians and serious DIYers.
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Subaru Service Manuals: You can purchase official Subaru service manuals for your specific model and year. These manuals contain detailed repair procedures, specifications, and diagrams.
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Technical Service Bulletins (TSBs): TSBs are issued by Subaru to address common problems or issues that may arise with their vehicles. TSBs provide information on how to diagnose and repair these issues. You can often find TSBs online through various automotive information websites or through the STIS portal.
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Online Forums and Communities: Numerous online forums and communities are dedicated to Subaru vehicles. These forums can be a valuable resource for information, as experienced owners and technicians often share their knowledge and experiences.
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ALLDATA and Mitchell OnDemand: These are subscription-based services that provide access to a wide range of automotive repair information, including Subaru-specific data. They compile information from various sources, including service manuals, TSBs, and wiring diagrams.
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Local Subaru Dealerships: Your local Subaru dealership’s service department can be a valuable resource for technical information. They have access to the latest diagnostic tools, software updates, and technical expertise.
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YouTube Channels and Video Tutorials: Many YouTube channels and websites offer video tutorials on diagnosing and repairing Subaru vehicles. These videos can provide visual guidance and step-by-step instructions.
3.2. What Are Common Subaru-Specific Codes?
Subaru-specific codes, often starting with “P1,” indicate issues unique to Subaru vehicles. These may include:
- P1443: Evaporative Emission Control System Vent Control Valve Circuit Malfunction
- P1507: Idle Control System Malfunction (Fail-Safe)
- P1137: Lack of HO2S Switch – Sensor Indicates Lean Condition (Bank 1 Sensor 2)
3.3. What Are Subaru-Specific Diagnostic Tools?
While a generic OBD-II scanner works for basic codes, Subaru-specific diagnostic tools offer advanced capabilities. These tools can access Subaru’s unique systems and provide more detailed information.
Examples include:
- Subaru Select Monitor (SSM): A dealer-level tool that offers comprehensive diagnostics.
- Autel MaxiSYS: A professional-grade scan tool with Subaru-specific functions.
- Launch X431: A versatile scan tool compatible with various makes, including Subaru.
4. What Are the Most Common Problems Triggering Subaru Check Engine Codes?
Identifying the most common problems that trigger Subaru check engine codes can streamline the diagnostic process. This includes understanding oxygen sensor issues, vacuum leaks, and catalytic converter problems.
4.1. How Do I Address Oxygen Sensor Issues?
Oxygen sensors monitor the amount of oxygen in the exhaust gas. Faulty sensors can cause a check engine light and affect fuel efficiency.
Here’s how to address oxygen sensor issues:
- Test the Sensor: Use a multimeter to test the sensor’s resistance and voltage.
- Inspect the Wiring: Check for damaged or corroded wires.
- Replace the Sensor: If the sensor is faulty, replace it with a new one.
- Check for Exhaust Leaks: Exhaust leaks near the sensor can affect its readings.
4.2. How Do I Fix Vacuum Leaks?
Vacuum leaks can disrupt the air
