What it means
The ECU continuously trims individual cylinders to equalise air-fuel ratio across the bank using ion-current or per-cylinder lambda feedback. For cylinder 12 the per-cylinder fuel correction has hit its software-side maximum or minimum without restoring balance, indicating the underlying mechanical or fuelling deviation is too large to compensate.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The ECU compares cylinder 12’s contribution with the other cylinders and adjusts its individual fuel correction to bring the air-fuel balance into line. P231F sets when that correction reaches its allowed maximum or minimum and cylinder 12 still remains out of balance.
Common causes
Most likely first.
- Fuel injector for cylinder 12 has drifted in flow rate
- Reduced compression on cylinder 12
Symptoms
- Check-engine light
- Rough idle
- Engine hesitation or reduced power
- Increased fuel consumption
How to diagnose it
- Use a scan tool to confirm P231F and review freeze-frame data and cylinder-balance or fuel-correction readings to see when cylinder 12 reached its limit.
- Check cylinder 12’s injector connector and wiring for loose pins, corrosion, or damage that could affect injector operation.
- Compare cylinder 12’s injector operation or flow with the other injectors using the manufacturer’s diagnostic procedure.
- Check cylinder 12 compression and compare it with the other cylinders to look for a mechanical cause of the imbalance.
Common fixes
- Repair a loose, corroded, or damaged connection at the cylinder 12 injector.
- Clean or replace the cylinder 12 injector if testing confirms restricted or incorrect flow.
- Repair the cause of low compression in cylinder 12 if a mechanical fault is confirmed.
Can I keep driving?
If the engine runs smoothly, limit driving and arrange diagnosis soon; rough running or hesitation can worsen and excess fuel or misfire can damage the catalytic converter. Stop driving if the misfire becomes severe or the check-engine light flashes.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.