P231C: Cylinder 9 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance

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.

Cylinder 9 is numbered according to the manufacturer: check your engine's manual before working on it.

Symptoms

• Check engine light on
• Rough idle, possibly misfire on cylinder 9
• Higher fuel consumption or exhaust smell

Most likely causes

• Injector of cylinder 9 delivering too much or too little (deposits, wear)
• Air leak at the intake port or injector seal of cylinder 9
• Valve of cylinder 9 not sealing (worn seat or guide), low compression
• Poor contact at the connector of injector 9

What to check, in order

Start with the simplest. You can tick off each step.

1. See whether the light is flashing: if so, reduce engine load and avoid driving
2. Inspect the spark plugs, coils, and connectors
3. Swap the coil with another cylinder and see if the misfire moves with it
4. Check the injector and look for air leaks
5. Run a compression test if everything above checks out

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P231C means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 3: Ignition and misfires
• 1C: Specific fault in that system

Frequently asked questions

Can I keep driving with code P231C?

Avoid driving if the light is flashing or the engine is shaking: unburned gasoline can damage the catalytic converter.

What causes code P231C?

The most common causes are: injector of cylinder 9 delivering too much or too little (deposits, wear); air leak at the intake port or injector seal of cylinder 9; valve of cylinder 9 not sealing (worn seat or guide), low compression.

Related codes

• P21A4Cylinder 9 Air-Fuel Ratio Imbalance
• P231BCylinder 8 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance
• P231DCylinder 10 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance
• P0309Cylinder 9 Misfire Detected
• P0209Cylinder 9 Injector A Circuit/Open
• P219ABank 1 Air-Fuel Ratio Imbalance
• P219BBank 2 Air-Fuel Ratio Imbalance

Sources

• law.cornell.edu: 13 CCR 1968.2
• static.nhtsa.gov: MC 10187790 9999
• static.nhtsa.gov: MC 10249505 0001

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.