P0312
Cylinder 12 Misfire Detected.
- Generic
- Powertrain
- Generic code
- High severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 3
- AreaIgnition system or misfire
- 12
- FaultThe specific fault in that area
What it means
P0312 means the engine control unit has detected misfires on cylinder 12 using crankshaft speed variation. The code is only relevant to engines with twelve cylinders, typically V12 or W12 layouts. The control unit saw the crankshaft slow down or accelerate unevenly after the cylinder 12 firing event, beyond its set limit, over a number of engine cycles.
Symptoms
- Engine running rough or shaking at idle
- Flashing or steady malfunction warning light
- Noticeable loss of power or hesitation under acceleration
- Smell of unburnt fuel from the exhaust
- Poor fuel economy
- Engine may enter a limp or reduced power mode
Possible causes
- Worn or faulty spark plug on cylinder 12likely
- Failed ignition coil or coil connector on cylinder 12likely
- Faulty or blocked fuel injector on cylinder 12, or its wiringpossible
- Vacuum or intake leak affecting the bank that includes cylinder 12possible
- Low cylinder compression from valve, piston ring or head gasket problemsrare
- Wiring or connector fault in the ignition or injector circuit for cylinder 12possible
- Outdated control unit software where a known misfire detection issue exists for the modelrare
How to diagnose
- Read all stored and pending codes and note any other misfire or fuel trim codes
- Review freeze frame data for engine load, speed, coolant temperature and fuel trims when the misfire set
- Check the engine misfire counters per cylinder to confirm the fault is isolated to cylinder 12
- Inspect the cylinder 12 coil, plug and injector connectors and wiring for damage, corrosion or oil contamination
- Remove and inspect the cylinder 12 spark plug for wear, fouling or damage, and compare with neighbouring cylinders
- Swap the cylinder 12 coil with a known good cylinder, clear the codes and see whether the misfire follows the coil
- Test injector operation and resistance on cylinder 12, and check for intake or vacuum leaks on that bank
- Carry out a compression or leak-down test if ignition and fuel checks find nothing; the procedure depends on the engine
How to fix
- Replace the spark plug on cylinder 12, and the full set if they are due or worn
- Replace the faulty ignition coil on cylinder 12
- Clean or replace the faulty fuel injector, or repair its wiring
- Repair any intake or vacuum leaks found on that bank
- Repair damaged wiring or connectors in the affected circuit
- Carry out the necessary mechanical engine repair if compression is low
Can I keep driving?
High severity
Avoid driving while the warning light is flashing, as raw fuel can overheat and damage the catalytic converter. If it is steady and the engine runs smoothly, drive gently and have it checked promptly.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.