Powertrain, generic (extended)

P231A Cylinder 7 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

P231A takes the series to cylinder 7; since P2310 to P2319 are ignition coil codes, the versions for cylinders 7 to 12 start at P231A. Only engines with eight or more cylinders can set it. Mercedes-Benz documents it for the 2022 to 2024 SL with the AMG M177 V8 (LI07.61-P-077382): the ME control unit reports irregular mixture formation at cylinder 7 and disables the injector quantity adjustment. The remedy is new ME software; if the latest version is already installed and P231A00 is still present, Mercedes asks for a technical case to be opened instead of parts being replaced. On other engines the causes match smaller ones: injector 7, an intake leak at that port, valve sealing. P21A2 and P0307 cover the same cylinder; the MIL is on.

Likely causes

Injector of cylinder 7 delivering too much or too little (deposits, wear)

Air leak at the intake port or injector seal of cylinder 7

Valve of cylinder 7 not sealing (worn seat or guide), low compression

ME engine software (Mercedes M177, LI07.61-P-077382)

Symptoms

Check engine light on
Rough idle, possibly misfire on cylinder 7
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 7intake manifoldcylinder compression

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