OBD2 Code Lookup: What Your Check Engine Light Means
When your check engine light comes on, your car stores a Diagnostic Trouble Code (DTC) — a short letter+number combo like P0420 or P0300. Plug in any cheap OBD2 scanner, read the code, then look it up below for a plain-English translation.
From code to fix in 3 steps
- Pull the code with any OBD2 scanner (or have an auto parts store read it free).
- Find the code below to understand what system it points to and how serious it is.
- Open your CaliperAI garage and ask the AI mechanic to walk through the fix on your exact year, make, model, and engine.
- P0420Moderate severityEmissions
Catalyst System Efficiency Below Threshold (Bank 1)
Your catalytic converter isn't cleaning exhaust gases as well as it should. Most often the converter itself is aging out, but a lazy downstream oxygen sensor or a small exhaust leak can trigger the same code.
Common causes
- Worn or failing catalytic converter
- Faulty downstream (post-cat) O2 sensor
- Exhaust leak before or at the converter
- Engine running rich from upstream fuel/ignition problem
- P0300High severityIgnition
Random / Multiple Cylinder Misfire Detected
The engine computer noticed cylinders firing unevenly across more than one cylinder. You'll usually feel a shake at idle, lose power, and see the check engine light flash on hard acceleration.
Common causes
- Worn spark plugs or ignition coils
- Vacuum leak (intake gasket, PCV hose)
- Weak fuel pump or clogged injectors
- Low compression in multiple cylinders
- P0171Moderate severityFuel/Air
System Too Lean (Bank 1)
Too much air is reaching the engine for the amount of fuel being injected. Almost always an unmetered air leak or a dirty/failing mass airflow sensor.
Common causes
- Vacuum leak (cracked intake boot, PCV valve)
- Dirty or failing MAF sensor
- Weak fuel pump or clogged fuel filter
- Leaking fuel injector seals
- P0174Moderate severityFuel/Air
System Too Lean (Bank 2)
Same idea as P0171 but on the opposite cylinder bank. When P0171 and P0174 appear together, suspect a shared problem like the MAF sensor or a big vacuum leak.
Common causes
- Large vacuum or intake leak
- MAF sensor contamination
- Failing fuel pump
- Exhaust leak ahead of the front O2 sensor
- P0128Low severityPowertrain
Coolant Temperature Below Thermostat Regulating Temperature
The engine takes too long to warm up. The fix is almost always a stuck-open thermostat, but a faulty coolant temp sensor or cold-climate driving can also trigger it.
Common causes
- Stuck-open thermostat
- Faulty engine coolant temperature (ECT) sensor
- Low coolant level
- P0442Low severityEmissions
Evaporative Emission System Small Leak Detected
The fuel vapor recovery system is leaking a tiny amount of pressure. The #1 cause is a loose, cracked, or worn gas cap.
Common causes
- Loose or failed gas cap
- Cracked EVAP hose
- Faulty purge or vent valve
- P0455Low severityEmissions
Evaporative Emission System Large Leak Detected
Same family as P0442 but a bigger leak. Start with the gas cap, then look for disconnected EVAP lines or a stuck-open vent valve.
Common causes
- Missing or badly worn gas cap
- Disconnected EVAP hose
- Failed vent control valve
- P0301High severityIgnition
Cylinder 1 Misfire Detected
Cylinder 1 specifically is misfiring. Swap the coil and spark plug from cylinder 1 with another cylinder — if the code follows the part, you found it.
Common causes
- Bad ignition coil on cylinder 1
- Worn spark plug
- Failed fuel injector
- Low compression on that cylinder
- P0302High severityIgnition
Cylinder 2 Misfire Detected
Same as P0301 but on cylinder 2. Diagnose the same way — swap coil and plug to confirm.
Common causes
- Bad ignition coil on cylinder 2
- Worn spark plug
- Failed fuel injector
- Mechanical issue (valve, compression) on that cylinder
- P0303High severityIgnition
Cylinder 3 Misfire Detected
Cylinder 3 is misfiring. Cylinder location depends on your engine layout — your CaliperAI chat can tell you exactly which one to pull for your vehicle.
Common causes
- Coil pack failure
- Fouled or worn spark plug
- Injector clog
- P0401Moderate severityEmissions
Exhaust Gas Recirculation (EGR) Flow Insufficient
The EGR system isn't moving enough exhaust back into the intake. Usually carbon buildup is clogging the EGR passages or the valve itself is stuck.
Common causes
- Clogged EGR passages (carbon)
- Stuck or failed EGR valve
- Failed DPFE / EGR position sensor
- P0430Moderate severityEmissions
Catalyst System Efficiency Below Threshold (Bank 2)
Identical to P0420 but on the second cylinder bank (V6/V8 engines). Same diagnostic flow — verify with the downstream O2 sensor before replacing the converter.
Common causes
- Worn catalytic converter (bank 2)
- Faulty downstream O2 sensor
- Exhaust leak
- P0440Low severityEmissions
Evaporative Emission Control System Malfunction
A general EVAP system fault — somewhere in the fuel vapor recovery loop there's a leak or component failure. Start with the gas cap.
Common causes
- Loose gas cap
- Cracked charcoal canister
- Failed purge solenoid
- P0700High severityTransmission
Transmission Control System Malfunction
The transmission computer has logged a problem and is asking the engine computer to turn on the check engine light. P0700 is the headline — there's a more specific transmission code stored alongside it.
Common causes
- Internal transmission fault (paired with another P07xx code)
- Low or burnt transmission fluid
- Failed shift solenoid
- P0011Moderate severityPowertrain
Camshaft Position 'A' Timing Over-Advanced (Bank 1)
Variable valve timing on the intake cam is advanced further than the computer expected. Most often dirty oil or a stuck VVT solenoid is to blame.
Common causes
- Low or dirty engine oil
- Failed VVT/oil control solenoid
- Worn timing chain
- P0606High severityPowertrain
ECM/PCM Processor Fault
The engine control module itself has reported an internal fault. Sometimes it's a bad ground or battery voltage issue rather than the computer itself.
Common causes
- Poor ECM ground connection
- Battery or charging system voltage out of spec
- Failed engine control module
Why generic code definitions aren't enough
A code like P0301 means "cylinder 1 misfire" on every car — but where cylinder 1 actually sits, which coil to swap, and what the torque spec is for the spark plugs depends on your specific engine. That's the gap CaliperAI fills: paste the code into your vehicle's chat and get the real repair steps for your car, not a generic forum post.