OBD-II Code P0312 — What It Means & How to Fix It

What Does This Error Mean?

P0312 means the engine computer detected a misfire in cylinder 12. This is the last cylinder in a V12 engine, and it's failing to ignite its fuel/air mixture properly. You'll likely notice rough running, a slight shudder at idle, or hesitation under acceleration. Misfires at any cylinder produce unburned fuel that harms the catalytic converter. This code is exclusive to 12-cylinder engines.

Affected Models

All vehicles 1996+ with 12 cylinders
Common in BMW 760i/760Li V12
Common in Mercedes-Benz S600/CL600 V12
Common in Audi A8 W12
Common in Ferrari and Lamborghini V12 models

Common Causes

Worn or carbon-fouled spark plug in cylinder 12 with an excessive electrode gap
Failed ignition coil on cylinder 12 that can no longer produce sufficient spark
Clogged or stuck-open fuel injector causing incorrect fuel delivery to cylinder 12
Vacuum leak at the intake manifold near cylinder 12 creating a lean misfire
Mechanical wear in cylinder 12 — low compression from worn rings or damaged valves

How to Fix It

Pull codes with an OBD-II scanner and check for any companion codes. If you have P0312 alone, the problem is isolated to cylinder 12. If you also see P0300 (random misfire), multiple cylinders may be affected and a system-wide cause like bad spark plugs is more likely.

A code P0300 alongside P0312 suggests replacing all spark plugs, not just one.
Locate and remove the spark plug for cylinder 12. V12 engines number cylinders bank by bank — cylinder 12 is at the end of one bank. Inspect the plug for heavy deposits, a cracked insulator, or a badly worn electrode. Replace as needed.

Always use the exact plug specification from your owner's manual — V12 engines can be sensitive to plug type.
Test the ignition coil using the swap method. Move the coil from cylinder 12 to a different cylinder, clear the code, and drive. If the misfire moves with the coil, replace the coil. If the misfire stays on cylinder 12, the coil is fine.

This test takes about 15 minutes and costs nothing.
Listen for fuel injector operation with a mechanic's stethoscope or screwdriver held to the injector body. A properly working injector makes a rapid clicking sound. Test the injector's resistance with a multimeter — it should read 10-17 ohms.

A cylinder 12 injector that is partially clogged may click but still cause a misfire.
If ignition and fuel checks come back clean, schedule a compression test at a mechanic. Low compression in cylinder 12 relative to the other cylinders indicates mechanical wear. A leak-down test can further identify whether the issue is rings, valves, or a head gasket.

V12 head gasket repairs can be very expensive — often $2,000-$5,000+ due to the labor involved.

When to Call a Professional

V12 engines in luxury vehicles often have very tight engine bays. Spark plug access on cylinder 12 — typically at the rear of the engine — can require removing intake components. If you're not comfortable with this level of disassembly, a shop visit is worthwhile. Internal compression issues are strictly professional repairs. Budget $300-$600 for a full tune-up on a V12.

Frequently Asked Questions

Is P0312 only for V12 engines?

Yes — P0312 only applies to vehicles with at least 12 cylinders. This means V12 or W12 engines. If a scanner shows P0312 on a car with fewer than 12 cylinders, verify the scan was done correctly. The code won't be generated by a V8 or smaller engine.

How urgent is P0312?

It's moderately urgent. One misfiring cylinder on a V12 means the engine is running on 11 cylinders. You'll notice reduced performance and fuel economy. More importantly, unburned fuel in the exhaust can overheat and damage the catalytic converters. Address it within a week or two — sooner if the check engine light is flashing.

Can weather cause a cylinder 12 misfire?

Cold, wet weather can trigger misfires on any cylinder. Moisture can cause spark plug wires or ignition coils to arc to ground rather than fire the plug. If the misfire only happens when it's cold or wet and goes away when the engine warms up, the coil or plug boot is the likely culprit.