Worn or fouled spark plug on cylinder 12, often from oil seepage or a rich mixture; Failed ignition coil or coil-on-plug boot specific to cylinder 12; Faulty fuel injector that is clogged, leaking, or not receiving its pulse signal; Vacuum leak or intake manifold gasket failure near cylinder 12 that leans out that runner; Low compression from a burned valve, worn rings, or a failed head gasket on that cylinder; Damaged wiring or connector at the cylinder 12 coil or injector harness; Mechanical valvetrain fault such as a collapsed lifter or worn cam lobe affecting cylinder 12

DTC codes / P03xx · Ignition system and misfire / P0312

P0312Cylinder 12 Misfire Detected

Q: What does code P0312 mean?

Cylinder 12 Misfire Detected. P0312 means the engine computer detected a misfire on cylinder 12, which is the twelfth cylinder in the engine's firing order and typically found only on V12 engines. The computer monitors crankshaft speed fluctuations and flags this code when cylinder 12 fails to contribute its expected power stroke, which can damage the catalytic converter if left unchecked.

P0312 means the engine computer detected a misfire on cylinder 12, which is the twelfth cylinder in the engine's firing order and typically found only on V12 engines. The computer monitors crankshaft speed fluctuations and flags this code when cylinder 12 fails to contribute its expected power stroke, which can damage the catalytic converter if left unchecked.

SystemPowertrain

Code typeGeneric (all makes)

SeverityHigh

Fix difficultyProfessional

AreaIgnition system and misfire

Symptoms you may notice

Often no symptoms besides the warning light, especially during light-throttle cruising
Noticeable engine shake or vibration at idle that may smooth out at higher rpm
A flashing check engine light during acceleration, indicating a misfire severe enough to harm the catalytic converter
Rough running or hesitation when the engine is under load, such as climbing a hill
A distinct exhaust note or puffing sound from the tailpipe consistent with a dead cylinder
Reduced fuel economy and a faint raw-fuel smell at the exhaust

Common causes, most likely first

Worn or fouled spark plug on cylinder 12, often from oil seepage or a rich mixture
Failed ignition coil or coil-on-plug boot specific to cylinder 12
Faulty fuel injector that is clogged, leaking, or not receiving its pulse signal
Vacuum leak or intake manifold gasket failure near cylinder 12 that leans out that runner
Low compression from a burned valve, worn rings, or a failed head gasket on that cylinder
Damaged wiring or connector at the cylinder 12 coil or injector harness
Mechanical valvetrain fault such as a collapsed lifter or worn cam lobe affecting cylinder 12

How to diagnose and fix it

Scan all stored and pending codes and record freeze-frame data to confirm the misfire is isolated to cylinder 12
Inspect the cylinder 12 spark plug and ignition coil, swapping the coil to another cylinder to see if the misfire follows
Perform a cylinder balance or power balance test to verify which cylinder is actually dead
Check fuel injector operation with a noid light and listen for injector clicking, then test injector resistance
Run a compression test and, if low, a leak-down test on cylinder 12 to rule out mechanical failure
Smoke-test the intake system for vacuum leaks near cylinder 12 and inspect the harness for chafing or corrosion
Repair the confirmed fault, clear the codes, and road-test to verify the misfire counter stays at zero

Cylinder 12 misfires that fool good technicians

A P0312 on a V12 often gets misdiagnosed as a coil problem when the real cause is a failed intake manifold gasket that leans out only the rear cylinders.
Some engines use cylinder deactivation, and a stuck lifter on a deactivated cylinder can set a misfire code that disappears once the engine warms up and reactivates that bank.
A marginal battery or charging system can cause random misfires on high-cylinder-count engines because the coil dwell time suffers when voltage drops under load.
The misfire counter can be triggered by a worn crankshaft reluctor ring or a failing crank sensor, making cylinder 12 look guilty when the signal itself is erratic.
Never replace the catalytic converter for a P0312 until the misfire is fixed, because unburned fuel from cylinder 12 will destroy the new converter in short order.

Can I keep driving with P0312?

Avoid driving with a flashing check engine light because raw fuel from the misfiring cylinder will overheat and ruin the catalytic converter. If the light is steady and the engine runs smoothly, you can drive gently to a repair shop, but keep the trip short and avoid heavy acceleration.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.