Cylinder 12 Misfire Detected
What it means
The engine computer has detected repeated misfires in cylinder 12, the last cylinder of a twelve-cylinder engine; the code does not appear on other engines. The cause is usually its spark plug, ignition coil or injector.
Symptoms4
- Check engine light on, possibly flashing under load.
- Slightly uneven engine running at idle.
- Minor loss of power.
- Fuel smell from the exhaust or higher consumption.
Possible causes6
- Worn or fouled spark plug in cylinder 12.
- Weak ignition coil for cylinder 12 or a cracked boot.
- Clogged or faulty injector for this cylinder.
- Damaged wiring or connector, especially at the end of the harness where heat is higher.
- Vacuum leak near the cylinder 12 intake runner.
- Low compression in the cylinder (less common).
How to diagnose5
- Locate cylinder 12 in the service manual, because the numbering across the two banks differs between makers.
- Watch the misfire counter with a scan tool that supports this engine, and compare with cylinder 11 and the rest of the bank.
- Inspect the plug, then swap the coil with another cylinder and see whether the misfire moves.
- Check the injector, its signal and its connector.
- Have a compression test done if the fault remains after that.
How to fix6
- Replace the spark plug, or the full set if it is due.
- Replace the ignition coil the misfire followed.
- Repair damaged wiring or connectors.
- Clean or replace the injector.
- Fix a vacuum leak or low compression if confirmed.
- Clear the codes, road test and confirm the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost starts with a plug or coil, but parts and access labor on twelve-cylinder engines are higher than usual, and the top of the range is reached with an injector.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
P0311 and P0312 appeared together. Should I replace two coils?
Not before testing. If both cylinders are on the same bank, the cause may be a shared connector or harness, or a vacuum leak near both.
Is this code dangerous for the engine?
The main risk is to the catalytic converter on that cylinder's bank from unburned fuel, so repair it quickly and stop if the check engine light flashes.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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