Engine misfires at idle
Rough idle, hunting RPM — how to start diagnosing and which OBD-II codes to check first.
Rough idle is one of the most common symptoms. In 80% of cases it's a misfire (P030x codes). In 15% — idle-air control or throttle body issues. The rest — compression or vacuum leaks.
First OBD-II step: read stored + pending DTCs. P0300 (random) → look for a global cause (spark plugs, MAF, fuel). P0301-P0308 → the number IS the cylinder — jump straight to that coil/plug/injector.
Live data worth watching: STFT/LTFT, MAF g/s at idle (spec: 3-5 g/s), engine RPM, coolant temp. LTFT > +15% at idle = lean condition (look for vacuum leak).
First diagnostic steps
- Read stored codes (Mode 03) and pending (Mode 07)
- Capture freeze frame at the moment the code set
- Inspect spark plugs and coils (if misfire is on one cylinder — swap coil to a different cylinder and see if the fault follows)
- Look for intake vacuum leaks (stethoscope or soapy water)
- Clean MAF sensor with MAF cleaner
Likely fault codes
Click a code for detailed description, causes, and diagnostic plan.