P231CCylinder 9 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P231C points to cylinder 9 and therefore exists only on ten- and twelve-cylinder engines, a small group of mostly high-performance and heavy-duty petrol units. The balancing logic has pushed the fuel trim for cylinder 9 to its limit. No manufacturer documents the code publicly and the NHTSA index has no bulletin for it. With five or six cylinders per bank, singling out one cylinder through the oxygen sensor is harder than on a four, so such engines lean on crankshaft speed analysis or active tests. Mercedes calls the underlying function injector quantity adjustment for cylinder balancing, and its M177 bulletin shows the series in use on large engines. CARB lets a maker store a bank code when a cylinder cannot be resolved. Check injector 9, intake sealing and compression, and compare P0309 and P21A4.
- What you may notice
- Check engine light on
- Rough idle, possibly misfire on cylinder 9
- Higher fuel consumption or exhaust smell
- Likely causes
- LikelyInjector of cylinder 9 delivering too much or too little (deposits, wear)
- PossibleAir leak at the intake port or injector seal of cylinder 9
- PossibleValve of cylinder 9 not sealing (worn seat or guide), low compression
- RarePoor contact at the connector of injector 9
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €0 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.