OBD-II library

Common diagnostic codes

Free guides for high-search trouble codes. Open a code for causes and test order — then continue in MasterMind locked to your vehicle.

• P0300Random / Multiple Cylinder MisfireP0300 means the engine control module detected misfires across more than one cylinder — or couldn’t pin the miss to a single cylinder. Shared problems beat one bad coil.
• P0302Cylinder 2 MisfireP0302 means cylinder 2 isn’t contributing cleanly. Unlike P0300, you can swap parts between cylinders and watch the code follow the failed piece.
• P0420Catalyst Efficiency Below Threshold (Bank 1)P0420 means bank 1 catalyst efficiency looks low to the computer. It can be the cat — or exhaust leaks, O2 issues, and engine problems that fake or destroy converters.
• P0171System Too Lean (Bank 1)P0171 means bank 1 is running lean — the computer is adding fuel to catch up. Unmetered air, MAF issues, and weak fuel delivery are the usual suspects.
• P0455EVAP System Large LeakP0455 means the evaporative emissions system saw a large leak during its self-test. Start with the gas cap, then hoses and valves — not a parts shotgun under the hood.