🔧 Navigate your Subaru Check Engine Codes

Complete Subaru CEL Code Guide

Understand what your Subaru’s check engine light really means. This guide breaks down the most common Subaru CEL (Check Engine Light) codes — with explanations, causes, and recommended fixes.

What Is a Subaru Check Engine Code?

When your Subaru’s onboard computer detects something out of spec, it stores a diagnostic trouble code (DTC). These CEL codes help identify problems like misfires, emissions faults, or sensor failures.

Even reliable models like the Subaru Forester, Outback, and Impreza can trigger CELs due to:

Aging components
Vacuum leaks
Sensor malfunctions
Emissions control issues

Why This Guide Is Different

ToolGrit’s Subaru code reference is designed for real-world diagnosis. Whether you’re a weekend DIYer or just want to understand what your mechanic’s talking about, this guide helps you:

Decode Subaru check engine light codes
Understand symptoms and causes
Learn which parts are usually involved
Get practical tips before you grab a wrench

Explore Common Subaru Codes

Use the searchable table below to filter by code, model year, part type, or description. Click any column to sort. Codes are tailored to Subaru’s most common issues — so you can focus on what’s relevant.

Most popular codes include:

P0420 – Catalyst System Efficiency Below Threshold
P0301 – Cylinder 1 Misfire Detected
P0457 – EVAP System Leak (Loose Gas Cap)

🔧 Recommended Tools for Subaru CEL Diagnosis

These affordable tools will help you read, diagnose, and fix check engine light issues on most Subaru models.

✅ OBD-II Scan Tool
Read Subaru-specific codes, monitor sensor data, and clear the check engine light.
➡️ Best if it supports live data and freeze frame.
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🖼️ Image Prompt: “modern OBD-II scanner with live data screen next to Subaru dashboard”

✅ MAF Sensor Cleaner
Safe spray for cleaning your mass airflow sensor — a common culprit in lean codes like P0171.
➡️ Don’t use regular electronics cleaner; this is designed for airflow sensors.
🔗 Buy MAF Cleaner
🖼️ Image Prompt: “can of MAF sensor cleaner on workbench with Subaru airbox”

✅ Oxygen Sensor Socket
Special socket makes O2 sensor removal easier, even with heat shields in the way.
➡️ Helpful for P0420, P0137, and P0141 repairs.
🔗 Get Sensor Socket
🖼️ Image Prompt: “oxygen sensor socket tool removing sensor on Subaru exhaust”

✅ Digital Multimeter
For checking voltage, resistance, and sensor signals like IAT, MAP, or O2 heater circuits.
➡️ Essential for deeper diagnostics and verifying wiring faults.
🔗 Shop Multimeters
🖼️ Image Prompt: “DIYer using a digital multimeter on Subaru engine harness”

💡 Heads-up: Some links may be affiliate links — no extra cost to you, but they help support ToolGrit.

Code	Description	Models/Years	Likely Causes	Parts Needed	Fix/Tips
P0420	Catalyst System Efficiency Below Threshold (Bank 1)	Forester (2006–2013), Outback (2005–2014)	Worn catalytic converter, exhaust leaks, bad O2 sensor	Catalytic converter, O2 sensors	Check for leaks before the cat; scan tool to compare O2 readings.
P0171	System Too Lean (Bank 1)	Forester (2006–2013), Outback (2005–2014)	Vacuum leak, dirty MAF, low fuel pressure	MAF sensor, fuel filter, vacuum lines	Inspect intake for leaks, clean/replace MAF, check fuel trims.
P0301	Cylinder 1 Misfire Detected	Forester (2006–2013), Outback (2005–2014)	Bad spark plug/coil, injector, low compression	Spark plug, coil, injector	Swap plug/coil, check injector, run compression test.
P0457	EVAP System Leak Detected (Fuel Cap Loose/Off)	Forester (2006–2013), Outback (2005–2014)	Loose cap, cracked hose, purge valve	Gas cap, purge valve	Tighten/replace cap, inspect lines and purge solenoid.
P0137	O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)	Forester (2006–2013), Outback (2005–2014)	Bad rear O2, exhaust leak, wiring fault	O2 sensor, harness	Replace sensor, check for rusted/cut wires.
P0113	Intake Air Temp Sensor 1 Circuit High Input	Forester (2006–2013), Outback (2005–2014)	Unplugged sensor, corrosion, open circuit	IAT sensor	Plug in/replace sensor if reading high with cold engine.
P0102	Mass or Volume Air Flow Circuit Low Input	Forester (2006–2013), Outback (2005–2014)	Dirty/failing MAF, intake obstruction	MAF sensor, air filter	Clean sensor and inspect filter/tubing for damage.
P0507	Idle Control System RPM Higher Than Expected	Forester (2006–2013), Outback (2005–2014)	Vacuum leak, dirty throttle body	Throttle body, PCV valve	Clean throttle and IAC; check hoses and PCV.
P2096	Post Catalyst Fuel Trim System Too Lean (Bank 1)	Forester (2006–2013), Outback (2005–2014)	Exhaust leak after cat, lazy O2	O2 sensor, manifold gasket	Replace sensor or fix post-cat leaks.
P0141	O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)	Forester (2006–2013), Outback (2005–2014)	Heater failure, blown fuse, broken wire	O2 sensor, fuse	Replace sensor, check heater circuit wiring.
P0302	Cylinder 2 Misfire Detected	Forester (2006–2013), Outback (2005–2014)	Same as P0301 but cylinder 2	Spark plug, coil, injector	Swap parts with known good cylinder to isolate.
P0303	Cylinder 3 Misfire Detected	Forester (2006–2013), Outback (2005–2014)	Same as P0301 but cylinder 3	Spark plug, coil, injector	Same test method as P0301—rotate plugs/coils and observe.
P0304	Cylinder 4 Misfire Detected	Forester (2006–2013), Outback (2005–2014)	Same as P0301 but cylinder 4	Spark plug, coil, injector	Final cylinder in sequence. Check compression if misfire persists.
P0118	Engine Coolant Temp Circuit High Input	Forester (2006–2013), Outback (2005–2014)	Unplugged or faulty sensor, open wiring	Coolant temp sensor	Check resistance vs temp. Replace if out of spec.
P0128	Coolant Thermostat Below Reg Temp	Forester (2006–2013), Outback (2005–2014)	Stuck open thermostat, low coolant, sensor error	Thermostat, coolant	Replace thermostat. Top off coolant and recheck.
P0421	Warm Up Catalyst Efficiency Below Threshold (Bank 1)	Forester (2006–2013), Outback (2005–2014)	Early cat failure or lazy O2 sensor	Catalytic converter, O2 sensor	Usually a weak converter or upstream O2. Scan for response time.
P0340	Camshaft Position Sensor Circuit Malfunction	Forester (2006–2013), Outback (2005–2014)	Failed sensor, loose connector, timing issue	Cam sensor, timing chain	Check sensor connection and cam timing.
P0442	EVAP Leak Detected (Small Leak)	Forester (2006–2013), Outback (2005–2014)	Small hose crack, loose cap, leak at purge valve	EVAP hose, gas cap	Use smoke test or scanner with EVAP test mode.
P0440	EVAP Emission Control System Malfunction	Forester (2006–2013), Outback (2005–2014)	General EVAP failure, purge valve or vent solenoid	Purge valve, vent valve, gas cap	Check all EVAP components for function. Scan for monitor readiness.
P0505	Idle Control System Malfunction	Forester (2006–2013), Outback (2005–2014)	IAC valve fault, vacuum leak, dirty throttle	IAC valve, hoses	Clean throttle, test IAC function, check for vacuum issues.