Driver triage
Check Engine Light Is On
A solid light means the car usually needs diagnosis soon. A flashing light means active misfire risk and should be treated as urgent.
Educational reference only, not professional repair advice. A fault code is a starting point, not a diagnosis. Vehicle symptoms, live data, service history, and manufacturer-specific procedures change the repair path. If the check engine light is flashing, the vehicle is overheating, brakes or steering feel unsafe, fuel odor or smoke appears, or the engine is running poorly, stop driving when safe and get professional help.
Quick triage
If you are unsure, treat the condition conservatively and have the vehicle checked.
Solid light
Avoid hard acceleration. Read the stored code as soon as practical — most solid lights allow continued driving short-term, but confirm with a scan.
Flashing light
Stop driving when safe and get professional help. A flashing light typically indicates an active misfire that can rapidly damage the catalytic converter and other components.
Diagnosis cost
Professional diagnosis often starts around $75–$150 before repairs. Many parts stores read codes at no charge.
Look up your code
Use a scanner or parts-store code readout, then search the code here.
P0420 Code: Catalyst System Efficiency Below Threshold
P0420 means the engine computer sees that the catalytic converter on bank 1 is not cleaning exhaust as efficiently as expected. The converter may be worn, but exhaust leaks, oxygen sensor problems, misfires, or fuel mixture issues can also
P0430 Code: Catalyst System Efficiency Below Threshold Bank 2
P0430 means the engine computer believes the catalytic converter on bank 2 is not reducing exhaust pollutants well enough. On V-style engines, bank 2 is the side that does not contain cylinder 1. The code can come from the converter itself,
P0171 Code: System Too Lean Bank 1
P0171 means the engine computer is adding extra fuel because bank 1 is running lean. In plain terms, the engine is getting too much air, not enough fuel, or incorrect sensor information. Vacuum leaks, intake leaks, fuel delivery issues, and
P0174 Code: System Too Lean Bank 2
P0174 means bank 2 is running lean and the computer is adding fuel to compensate. It is common on V-style engines when unmetered air enters one side of the intake, fuel delivery is weak, or the mass airflow reading is inaccurate for the act
P0300 Code: Random or Multiple Cylinder Misfire
P0300 means the engine is misfiring randomly or across multiple cylinders instead of one clearly identified cylinder. Ignition, fuel, air leaks, compression problems, or timing issues can all interrupt combustion and trigger this code.
P0301 Code: Cylinder 1 Misfire Detected
P0301 means cylinder 1 is not burning its air-fuel mixture consistently. The problem may be isolated to that cylinder, such as a spark plug, coil, injector, compression issue, or local air leak near the intake runner.