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Check engine light codes, explained
Plain-English guides to the most common OBD-II diagnostic trouble codes. What they mean, how serious they are, and what repairs actually cost.
P0420Catalyst System Efficiency Below Threshold
Moderate
Your catalytic converter isn't working efficiently. Learn what causes it, whether you can still drive, and what repairs cost ($100-$2,500).
Read full guide →P0171System Too Lean (Bank 1)
Moderate to Serious
Your engine is getting too much air or not enough fuel. Learn common causes, repair costs ($80-$1,000), and whether it's safe to drive.
Read full guide →P0300Random/Multiple Cylinder Misfire
Serious
Multiple cylinders are misfiring. This is serious. Learn causes, symptoms, repair costs ($150-$3,000+), and whether it's safe to drive.
Read full guide →P0128Coolant Thermostat Below Regulating Temperature
Low to Moderate
Your engine isn't warming up fast enough, usually due to a stuck thermostat. Learn causes, costs ($100-$500), and if it's safe to drive.
Read full guide →P0442EVAP System Leak Detected (Small Leak)
Low
A small leak in your EVAP system -- often just a loose gas cap. Learn causes, repair costs ($0-$600), and why this code is rarely urgent.
Read full guide →Got a check engine light?
Scan your car in 30 seconds. MechanicAI reads your codes, explains them in plain English, and tells you what repairs should cost.
