Diagnostics
Diagnostic walkthroughs, tool picks and repair cost breakdowns written for US DIY car owners.
P0171 and P0174 Codes: Causes, Fixes, and Costs
The P0171 and P0174 OBD2 codes mean the engine computer is detecting a lean air-fuel mixture on Bank 1 and Bank 2, respectively. When both codes appear together, common suspects include vacuum leaks, unmetered air, a dirty or faulty mass airflow sensor, and fuel delivery problems, so diagnose the shared cause before replacing parts.
P0430 Code: Causes, Symptoms, and Repair Costs
The P0430 OBD2 code means the engine computer has detected catalyst system efficiency below the expected threshold on Bank 2. A deteriorating catalytic converter is a common possibility, but exhaust leaks, oxygen sensor faults, engine misfires, and fuel mixture problems should be checked before replacing expensive parts.
P0420 Code: Causes, Symptoms, and Repair Costs
The P0420 OBD2 code means the engine computer has detected that the catalytic converter system on Bank 1 is not meeting its expected efficiency threshold. A failing catalytic converter is one possible cause, but exhaust leaks, oxygen sensor problems, engine misfires, and fuel mixture issues can also trigger this code.
P0300 Code: Random or Multiple Cylinder Misfire Explained
The P0300 OBD2 code means your engine computer has detected random or multiple-cylinder misfires, which can cause rough idling, hesitation, reduced power, and increased emissions. Common causes include worn spark plugs, faulty ignition coils, fuel delivery problems, and vacuum leaks, but proper diagnosis is needed before replacing parts.