OBD2 Trouble Code Library

Plain English explanations for the most common check engine light codes. Causes, symptoms, repair costs, and step-by-step fixes.

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P0300
Random/Multiple Cylinder Misfire Detected
P0300 means your engine is randomly misfiring — the combustion in one or more cylinders isn't completing properly. Unlik…
DIY: $20–$80 (spark plugs + coil packs)  ·  Shop: $150–$500+ depending on root cause
P0301
Cylinder 1 Misfire Detected
P0301 means your engine's cylinder 1 isn't firing correctly. The ECU detected that combustion in cylinder 1 is incomplet…
DIY: $15–$60 (spark plug + coil for cylinder 1)  ·  Shop: $120–$400 depending on cause
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means the catalytic converter on Bank 1 isn't reducing emissions efficiently enough. Your ECU monitors the oxygen …
DIY: $80–$200 (oxygen sensor replacement to rule out)  ·  Shop: $800–$2,500+ (catalytic converter replacement)
P0171
System Too Lean (Bank 1)
P0171 means the engine is getting too much air and not enough fuel on Bank 1. The ECU has maxed out its fuel trim correc…
DIY: $10–$80 (MAF cleaner, vacuum hose, injector cleaner)  ·  Shop: $150–$600 depending on root cause
P0304
Cylinder 4 Misfire Detected
P0304 means your engine's cylinder 4 is not firing correctly. The ECU detected incomplete or absent combustion in cylind…
DIY: $10–$70 (spark plug + coil)  ·  Shop: $120–$500 depending on root cause
P0455
Evaporative Emission Control System Leak Detected (Large Leak)
P0455 means the EVAP (Evaporative Emission Control) system has a large fuel vapor leak. Your ECU runs a periodic pressur…
DIY: $0–$40 (gas cap or hose)  ·  Shop: $150–$700 (smoke test + vent valve or filler neck)
P0128
Coolant Temperature Below Thermostat Regulating Temperature
P0128 means the engine coolant temperature is not reaching the ECU's expected operating temperature within the expected …
DIY: $15–$50 (thermostat replacement)  ·  Shop: $150–$300 (thermostat + coolant flush + labor)
P0131
O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
P0131 means the upstream oxygen sensor on Bank 1 (Sensor 1 — before the catalytic converter) is outputting voltage below…
DIY: $25–$80 (upstream O2 sensor)  ·  Shop: $150–$350 (sensor + labor)
P0446
Evaporative Emission System Vent Control Circuit Malfunction
P0446 means the ECU detected a malfunction in the EVAP (Evaporative Emission Control) vent control valve circuit. The ve…
DIY: $25–$80 (vent control valve)  ·  Shop: $150–$400 (valve replacement + diagnosis)
P0172
System Too Rich (Bank 1)
P0172 means the engine is getting too much fuel relative to air on Bank 1 — the opposite of P0171 (too lean). The ECU ha…
DIY: $10–$80 (MAF cleaner, filter, injector cleaner, coolant temp sensor)  ·  Shop: $150–$500 depending on root cause
P0401
Exhaust Gas Recirculation Flow Insufficient Detected
P0401 means the engine's Exhaust Gas Recirculation (EGR) system isn't circulating enough exhaust gas back into the intak…
DIY: $10–$60 (EGR cleaner or DPFE sensor)  ·  Shop: $150–$400 (EGR valve replacement + cleaning)
P0507
Idle Control System RPM Higher Than Expected
P0507 means the engine idle speed is running higher than the ECU's target RPM — typically the engine idles above 200 RPM…
DIY: $10–$80 (throttle body cleaner + IAC valve)  ·  Shop: $150–$450 (IAC valve or throttle body replacement + labor)
P0340
Camshaft Position Sensor Circuit Malfunction (Bank 1)
P0340 means the ECU detected a problem with the camshaft position sensor (CMP) circuit on Bank 1. The CMP sensor tells t…
DIY: $15–$80 (camshaft position sensor)  ·  Shop: $150–$400 (sensor replacement + labor)
P0442
Evaporative Emission Control System Leak Detected (Small Leak)
P0442 means the EVAP (Evaporative Emission Control) system has a small fuel vapor leak somewhere. This system captures f…
DIY: $5–$30 (new gas cap)  ·  Shop: $150–$600 (smoke test + component replacement)
P0430
Catalyst System Efficiency Below Threshold (Bank 2)
P0430 is the Bank 2 counterpart to the common P0420 code — it means the catalytic converter on Bank 2 isn't reducing emi…
DIY: $80–$200 (oxygen sensor replacement to rule out)  ·  Shop: $800–$$2,500+ (catalytic converter replacement)
P0174
System Too Lean (Bank 2)
P0174 means the engine is getting too much air and not enough fuel on Bank 2. The ECU has maxed out its fuel trim correc…
DIY: $10–$80 (MAF cleaner, vacuum hose, fuel filter)  ·  Shop: $150–$600 depending on root cause
P0456
Evaporative Emission Control System Small Leak Detected
P0456 means your EVAP (Evaporative Emission Control) system has a very small fuel vapor leak. It's similar to P0442 (sma…
DIY: $0–$40 (gas cap or hose)  ·  Shop: $150–$600 (smoke test + component replacement)
P0440
Evaporative Emission Control System Malfunction
P0440 is the general or "parent" EVAP (Evaporative Emission Control) system code. Unlike P0442 (small leak) or P0455 (la…
DIY: $0–$40 (gas cap or hose)  ·  Shop: $150–$700 (smoke test + canister or purge valve)

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