Diagnostics & OBD2 Codes
Look up fault codes, see plain‑English explanations and link into car‑specific advice.
Before you chase the code
- Record freeze-frame (RPM, load, coolant temp, fuel trims). It tells you when the fault happened.
- Don’t parts-dart: many codes are symptoms (vacuum leaks, wiring, low fuel pressure) not “replace this part”.
- Look for the upstream cause: a misfire or air leak can trigger catalyst / O2 / boost faults.
- Make it car-specific: the same code behaves differently depending on engine, sensors and ECU strategy.
Popular OBD2 Codes
- P0300 – Random/Multiple Cylinder Misfire
- P0171 – System Too Lean (Bank 1)
- P0299 – Turbo/Supercharger Underboost
- P0420 – Catalyst System Efficiency Below Threshold
- P0441 – EVAP System Incorrect Purge Flow
- P0456 – EVAP System Small Leak Detected
- P0430 – Catalyst Efficiency Below Threshold (Bank 2)
- P0401 – EGR Flow Insufficient
- P2453 – DPF Differential Pressure Sensor Range/Performance
- P0135 – O2 Sensor Heater Circuit (Bank 1 Sensor 1)
- P0101 – MAF Circuit Range/Performance
- P2015 – Intake Manifold Runner Position/Range