Trouble Codes

P0171 system too lean, bank 1: how to read fuel trims, find vacuum leaks, check the MAF sensor and fuel pressure, and confirm the fix before you buy parts.

P0300 random misfire code explained: what the engine computer counts, the order to check plugs, coils, vacuum leaks and fuel, and when to stop driving.

P0420 code explained: what catalyst efficiency below threshold means, the checks to run before replacing the cat, and the converter rules that apply.