Powertrain, generic (extended)

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

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

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.

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€0–1,500

Time0.5–6 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel injector cyl 9intake manifoldcylinder compression

References: law.cornell.edu, static.nhtsa.gov, static.nhtsa.gov.