Plain-English guides for car OBD-II check engine codes. Type a code below to find out what's wrong and what to do next.
OBD-II check engine codes explained in plain English
P0420 means the catalytic converter on Bank 1 (driver's side on most vehicles) isn't cleaning exhaust gases efficiently enough. It does not automatically mean the converter needs replacement — faulty oxygen sensors, exhaust leaks, misfires, and rich-running conditions can all trigger P0420 on an otherwise functional converter. Diagnosis comes first.
P0171 means the engine is running lean on Bank 1 — too much air or too little fuel in the mixture. The ECM detected this through oxygen sensor feedback and confirmed it by watching fuel trims climb to their limit. Common causes are vacuum leaks, a dirty MAF sensor, and fuel delivery issues. The cause depends heavily on where the lean condition occurs — at idle, at cruise, or under hard acceleration.
P0300 means one or more cylinders are misfiring randomly — combustion is not completing correctly. A flashing check engine light with P0300 means stop driving immediately: active misfires send raw fuel into the catalytic converter and can destroy it in minutes. With a steady light, limit driving and diagnose promptly. Common causes are worn spark plugs, failed ignition coils, low compression, and vacuum leaks.
P0455 means the EVAP system detected a large leak in the fuel vapor containment system. Despite the word 'large,' this is often caused by something as simple as a loose or improperly seated gas cap. The car will run normally, but it will fail an emissions test. Compare: P0442 is a small leak, P0455 is a large leak, P0456 is a very small leak.
P0442 means a small leak was detected in the EVAP (fuel vapor) system. It's often caused by a loose, worn, or damaged gas cap — but can also involve EVAP hoses, the purge valve, vent valve, charcoal canister, or a fuel tank seal. It's not an immediate breakdown risk, but the car may fail an emissions test.
There's a very small leak in the fuel vapor system — even smaller than a P0442.
The catalytic converter on the passenger side of the engine isn't working efficiently enough.
P0401 means insufficient EGR (Exhaust Gas Recirculation) flow. The system isn't routing enough exhaust gas back into the engine as commanded. Common causes include clogged EGR passages or valve, vacuum or control issues, wiring faults, or sensor feedback problems.
Cylinder 1 isn't firing correctly — combustion isn't happening as it should in that specific cylinder.
Cylinder 2 isn't firing correctly — combustion is incomplete or absent in that specific cylinder.
Cylinder 3 is misfiring — that cylinder isn't completing combustion the way it should.
The heater element inside the upstream oxygen sensor on the driver's side isn't working properly.
The heater inside the downstream oxygen sensor (after the catalytic converter) on the driver's side isn't working.
The mass airflow sensor is sending readings to the ECU that are outside the expected range for current driving conditions.
The intake air temperature sensor is sending a voltage signal that's higher than what the ECU expects, suggesting a cold or open circuit.
Plain-English guides to understand your car's diagnostic system
How car diagnostic codes work and what they mean when your check engine light turns on.
P0420, P0171, P0300, and more — the codes you're most likely to see and what usually causes them.
A flashing light means stop driving now. A solid light means schedule a diagnosis. Know the difference.
Yes — and it's the easiest fix. Learn which EVAP codes it triggers and when the light clears.
Fix the cause first, then clear with a scanner. Avoid the battery disconnect pitfall before emissions tests.
Why clearing codes before an emissions test can cause you to fail even with no active faults.
It depends on the code and whether the light is solid or flashing. Here's how to decide.
A step-by-step guide to reading, interpreting, and clearing codes with a basic scanner.
Which side of the engine is Bank 1? Which cylinder is Bank 2? Clear answers with diagrams.
Read the code, check freeze frame, inspect before replacing. The full diagnostic workflow in one place.
What each code state means, why permanent codes won't clear with a scanner, and how they affect emissions tests.
The snapshot your scanner captures when a code sets — engine speed, load, fuel trims, coolant temp, and more.
What positive and negative fuel trim numbers mean, and how they help diagnose vacuum leaks, MAF issues, and fuel delivery problems.
Flashing light? Stop now. Solid light? It depends. How to judge urgency from symptoms and code type.
Find This Code provides plain-English explanations of OBD-II diagnostic codes to help you understand what your check engine light is telling you. Our guides explain what each code means, its common causes, and typical repair costs so you're informed before heading to a mechanic.