Powertrain, generic (extended)

P231B Cylinder 8 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

P231B is the balance-limit code for cylinder 8, the last cylinder of a V8 and an inner one on V10 and V12 engines. The fuel trim for that cylinder has hit its stop while balancing. No maker documents P231B publicly; the nearest evidence is Mercedes' bulletin for the AMG M177 V8 in the SL, which shows that this engine uses the cylinder 7 to 12 range and treats it partly as a software issue, together with CARB's rule that the stored code should locate the cause as precisely as the method allows. On a V8, cylinder 8 shares a bank with three others, so cross-check the bank code P219A or P219B, the misfire counter P0308 and the imbalance code P21A3. Injector 8, the intake gasket and compression come next.

Likely causes

Injector of cylinder 8 delivering too much or too little (deposits, wear)

Air leak at the intake port or injector seal of cylinder 8

Valve of cylinder 8 not sealing (worn seat or guide), low compression

Poor contact at the connector of injector 8

Symptoms

Check engine light on
Rough idle, possibly misfire on cylinder 8
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 8intake manifoldcylinder compression

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