Fix P0312: Mechanic’s solutions that WORKED

The P0312 is one of several misfire codes you can read on your scan tool. I studied 120+ different vehicles with misfires and how were they fixed, to provide you with a list of the most common causes, also with an action plan to diagnose this fault in no time.

You don’t want to delay fixing your P0312 as the misfire will damage your catalytic converters or engine internals. Both of these fixes are priced at thousands of dollars while the misfire is often caused by a worn-out $5 spark plug. Fix it ASAP.

P0312 quick overview

Meaning	P0312: Cylinder #12 Misfire Detected
Is it serious?	Yes, this code is serious and can lead to severe engine damage if not addressed promptly.
Possible causes	-Faulty spark plug in cylinder 12 -Worn or damaged ignition coil for cylinder 12 -Fuel injector malfunction in cylinder 12 -Vacuum leaks affecting cylinder 12 -Fuel system issues (e.g., low fuel pressure) -Internal engine problems (e.g., valve or timing issues) -Wiring issues or poor electrical connections related to cylinder 12
How to diagnose?	Check Spark Plug: Inspect the spark plug in cylinder 12 for wear or damage. Test Ignition Coil: Swap the ignition coil from cylinder 12 with another cylinder to see if the misfire moves. Inspect Fuel Injector: Swap the fuel injector from cylinder 12 with another cylinder and monitor for changes. Perform Compression Test: Conduct a compression test on cylinder 12 to check for internal engine issues. Check for Vacuum Leaks: Inspect for vacuum leaks around cylinder 12, especially in the intake manifold. Review Wiring and Connections: Examine the wiring and connectors leading to cylinder 12 for damage or corrosion.

Case study: real cases of solving P0309 – P0312

These codes can be seen only on V10 and V12 engines since they are pointing to misfire on cylinders #9 – 12. There were not many service posts about them, so I threw these codes together into research to understand what is causing misfires in these giant engines most often.

How to fix P0309 on BMW

Car: 1999 BMW 750iL 5.4L
Problem: P0303, P0309

Diagnosing: Inspecting the spark plugs on cylinders #3 and #9 didn’t show anything unusual. The smoke test didn’t detect anything, but only because was done only with the engine warmed up. Repeating it on a cold engine showed a vacuum leak.
Fix: Replace intake gaskets.

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How to fix P0309/P0312 on another BMW

Car: 2003 BMW 760Li 6.0L
Problem: MIL LAMP ON, P0303, P0309, P0444

Diagnosing: Misfire on cylinders #3 and #9 seems to be common to BMW according to the last case. This time the technician changed the fuel pump which didn’t help and the customer took his car to the BMW dealership where they replaced all fuel injectors.
Fix: Replace Fuel injectors

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How to fix P0309/P0312 on Mercedes

Car: 1997 Mercedes-Benz SL600 6.0L
Problem: Misfire, Hesitation, P0300, P0309, P0312, P1580, P0507

Diagnosing: The advantage of diagnosing a car with multiple engine heads is to swap components between these heads to see if the problem moves to other side of the engine. The technician swapped throttle bodies which moved the misfire to another bank, confirming it is faulty.
Fix: Replace the throttle body

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How to fix P0309 on another Mercedes

Car: 2001 Mercedes-Benz S600 5.8L
Problem: Excessive Emissions, Misfire, MIL Lamp on, P0300, P0305, P0309, P03012, P208F

Diagnosing: Again the fault is in one bank according to cylinder numbers (5, 9, 12) but it wasn’t the throttle body. This time the technician went straight to exhaust, removing O2 sensors first which didn’t change much. After removing the catalytic converter the car immediately started to run better. The misfire was originally caused by ignition coils according to the technician.
Fix: Replace the ignition coils and catalytic converters.

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How to fix P0309 on Another Mercedes

Car: 2001 Mercedes-Benz S600 5.8L
Problem: Misfire, Check engine light, P0300, P0301, P0308, P0309, P3010

Diagnosing: Mercedes vehicles with V12 seemed to run into misfire issues often. This one was solved by replacing the ignition control module with the oxygen sensors.
Fix: Oxygen sensors/ignition control module

How to fix P0310 on Ford

Car: 2000 Ford Excursion Limited 6.8L
Problem: P0300, P0310, misfire under load

Diagnosing: The technician was able to hear noise coming from the alternator and a simple running test with the removed serpentine belt confirmed that it is causing a misfire under load.
Fix: Replace the alternator.

How to fix P0312 on BMW

Car: 2004 BMW 760Li 6.0L
Problem: P03012, cuts out,

Diagnosing: Changing the spark plug and ignition coil on the affected cylinder partially helped solve the misfire, but the car would still cut out occasionally. The car was taken to the dealership where they found a fuel pump wiring issue. Pretty good for the official dealership, usually they would replace half car before finding the wiring fault.
Fix: Replace the fuel pump wiring.

How to fix P0300 – P0312 on BMW

Car: 1996 BMW 750iL 5.4L
Problem: P0300 – P0309, P0310, P0311, P0312

Diagnosing: This problem was all about air intake. First, the intake gaskets needed to be changed for a small vacuum leak and the throttle body had to be cleaned and readjusted.
Fix: Throttle body settings

The key takeaways from these cases

• Codes P0309, P03010, P0311, and P0312 can be seen only on the biggest V10 and V12 engines. Make sure to diagnose it right as 12 ignition coils/spark plug won’t be cheap