P231B code meaning
Cylinder 8 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance
Powertrain system Generic code P23xx codes Check engine light
What P231B means
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.
P231B at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 6 hours
- Clears after repairAfter a drive cycle
Symptoms of P231B
- Check engine light on
- Rough idle, possibly misfire on cylinder 8
- Higher fuel consumption or exhaust smell
Common causes of P231B
- Most likelyInjector of cylinder 8 delivering too much or too little (deposits, wear)
- PossibleAir leak at the intake port or injector seal of cylinder 8
- PossibleValve of cylinder 8 not sealing (worn seat or guide), low compression
- Less likelyPoor contact at the connector of injector 8
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Fuel injector cyl 8Intake manifoldCylinder compression
What each part of P231B means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 3 | Area: ignition system or misfire. |
| 1B | The exact fault in that area. |
What to do next
- Note P231B and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P231B
What does P231B mean?
P231B means "Cylinder 8 Air-Fuel Ratio Imbalance - Adjustment At Limit During Balance". P231B is the balance-limit code for cylinder 8, the last cylinder of a V8 and an inner one on V10 and V12 engines. It is a generic code, so it means the same on every make.
Is it safe to drive with P231B?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P231B fail an emissions test?
Yes, it can. P231B is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P231B to fix?
Difficulty: medium. A typical repair takes 0.5 to 6 hours of work. This is a job for a workshop.
Will P231B clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.