OBD codes / P0312
P0312
SeriousCylinder 12 Misfire Detected
Treat P0312 with urgency despite how smooth the engine may feel. A steady light allows careful short-term driving; a FLASHING light means raw fuel is actively reaching the catalytic converter — back off and get it diagnosed or towed. V12 exhaust hardware is expensive enough that deferring a plug or coil is a bad trade.
What this code means
P0312 flags a misfire in cylinder 12, the last cylinder in a V12's firing order — so this code appears exclusively on twelve-cylinder engines in luxury and performance vehicles. The cylinder isn't burning its mixture reliably, and the most likely causes are its spark plug or ignition coil, followed by a weak or clogged fuel injector and, least often, low compression from a valve or ring problem. The proven test is swapping the suspect coil or plug to a neighboring cylinder and seeing whether the misfire code moves with it. Because a V12 runs almost smoothly on eleven cylinders, the symptom can be nothing more than a check-engine light — but the unburned fuel from the dead cylinder still cooks the catalytic converter on that bank. Cylinder 12's physical location varies by manufacturer, and on some engines the rearmost cylinders are the hardest to reach.
Symptoms you might notice
- Mild roughness or vibration, often barely perceptible
- Reduced power or hesitation under load
- Flashing check-engine light during an active misfire
- Reduced fuel economy
- Fuel smell from the exhaust in severe cases
What it costs to fix
Varies by vehicle — get it diagnosed.
The repair cost for this code depends too much on your exact vehicle and the underlying cause to quote a meaningful range. Confirm the cause first so you only pay for the fix you actually need.
Diagnose P0312 for my vehicle →Frequently asked
Which cars can even set P0312?
Only twelve-cylinder engines — V12s in high-end luxury and performance cars. On anything smaller, this code would be spurious: re-scan and verify before spending any diagnostic effort on it.
Can I drive with P0312?
With a steady light and no drivability change, careful short-term driving is usually fine while you book the repair. If the light flashes, the misfire is actively damaging the catalytic converter — reduce load immediately and get it seen.
Why is fixing one cylinder on a V12 sometimes expensive?
The parts are ordinary — a plug, a coil — but V12 engine bays are packed, and reaching some cylinders means removing covers, ducting, or other components. Labor to access the cylinder, not the part, is usually the bulk of the bill.
How do I confirm whether it's the coil or something else?
Swap cylinder 12's coil with a neighbor, clear the codes, and drive. If the misfire code follows the coil to the new cylinder, replace the coil; if P0312 stays put, test the plug, then the injector, then compression — in that order of likelihood.
Seeing P0312 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0312 for my vehicleRelated trouble codes
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.