P0312
Fix within a few daysCylinder 12 Misfire Detected
Code P0312 means Cylinder 12 Misfire Detected. Common causes: Ignition coil (40%), spark plug (35%), fuel injector (15%). Drivability: Limit driving, fix within a few days. Repair cost: $50-$200 DIY, $150-$500 professional.
Can I Drive With P0312?
Limit driving, fix within a few days
Essential trips under 10 miles, avoid highways and heavy acceleration. Do not drive if the Check Engine Light is flashing.
Stop driving immediately if: Flashing Check Engine Light, loud knocking noises, burning smell, significant power loss, or rotten egg smell from the exhaust.
Symptoms of P0312
- 1Engine runs rough or idles poorly
- 2Reduced engine power and acceleration
- 3Engine hesitation or stumbling
- 4Check Engine Light is on and may be flashing
- 5Decreased fuel economy
- 6Strong smell of raw fuel from the exhaust
Common Causes of P0312
Parts most likely causing this code, ranked by probability
A faulty ignition coil fails to provide adequate spark to cylinder 12, causing a misfire.
40% of P0312 cases are fixed by replacing the ignition coil
A worn, fouled, or damaged spark plug in cylinder 12 cannot ignite the fuel-air mixture effectively.
35% of P0312 cases are fixed by replacing the spark plug
A clogged or faulty fuel injector in cylinder 12 delivers too little or too much fuel, preventing proper combustion.
15% of P0312 cases are fixed by replacing the fuel injector
Damaged wiring or a loose connector to the spark plug, ignition coil, or fuel injector for cylinder 12 can interrupt signals.
5% of P0312 cases are fixed by repairing wiring or connectors
Low compression in cylinder 12, often due to worn piston rings, valves, or a head gasket leak, prevents proper combustion.
4% of P0312 cases are fixed by addressing engine compression issues
A rare internal fault in the PCM can incorrectly report a misfire or fail to control cylinder 12's ignition or fuel.
1% of P0312 cases are fixed by replacing the PCM
Check These First (Before Buying Parts)
- Visually inspect the ignition coil and spark plug for cylinder 12 for any obvious signs of damage, cracks, or corrosion.
- Check the electrical connector to the ignition coil and fuel injector for cylinder 12 to ensure it is securely seated and free of corrosion.
- Listen for any obvious vacuum leaks around the intake manifold that could affect a single cylinder.
How to Diagnose P0312
- 1Visually inspect the ignition coil and spark plug for cylinder 12 for any obvious damage, cracks, or corrosion.
- 2Swap the ignition coil from cylinder 12 with a known good cylinder (e.g., cylinder 11). If the misfire moves to the new cylinder, replace the coil.
- 3Swap the spark plug from cylinder 12 with a known good cylinder. If the misfire moves, replace the spark plug.
- 4Check the electrical connector and wiring to the ignition coil and fuel injector for cylinder 12 for damage or poor connection.
- 5Test the fuel injector for cylinder 12 using a noid light or by swapping it with another cylinder to see if the misfire moves.
- 6Perform a compression test on cylinder 12 to check for low compression, indicating an internal engine issue.
- 7Use a scan tool to monitor live data, specifically misfire counters for all cylinders, to confirm the misfire is isolated to cylinder 12.
P0312 on Popular Vehicles
BMW 7 Series
2003-2024P0312 is specific to V12 engines. On BMW V12s, ignition coils and spark plugs are common failure points due to heat and age. Fuel injectors can also fail, and vacuum leaks are a known issue that can cause misfires.
75% fixed by ignition coil or spark plug replacement
Mercedes-Benz S-Class
2000-2024For V12 S-Class models (e.g., S600, S65 AMG), P0312 often points to a failing ignition coil pack or spark plug. These engines are sensitive to proper maintenance. Less common but possible causes include wiring harness issues or fuel delivery problems.
70% fixed by ignition coil or spark plug replacement
Aston Martin
2004-2024Aston Martin V12 engines (e.g., DB9, Vanquish) can trigger P0312. Ignition coils and spark plugs are the primary suspects. Due to the engine layout, accessing cylinder 12 can be more involved, increasing labor costs.
80% fixed by ignition coil or spark plug replacement
Frequently Asked Questions
What does P0312 mean?
P0312 indicates that your vehicle's engine computer has detected a misfire specifically in cylinder 12. This means cylinder 12 is not contributing its full power to the engine's operation.
Can I drive with P0312?
Limit driving, fix within a few days. Driving with a P0312 code, especially if the Check Engine Light is flashing, can cause serious damage to your catalytic converter due to unburned fuel entering the exhaust. Drive only for essential, short trips until repaired.
Will P0312 clear itself?
No, P0312 will not clear itself. A misfire indicates a persistent problem that requires diagnosis and repair. The code will remain stored until the underlying issue is fixed and the system monitors complete a successful drive cycle.
How do I fix P0312 myself?
Start by inspecting the spark plug and ignition coil for cylinder 12. You can swap the coil with another cylinder to see if the misfire moves. If not, replace the spark plug. If the issue persists, you may need to test the fuel injector or perform a compression test.
How much does it cost to fix P0312?
The cost to fix P0312 varies. DIY costs are typically $50-$200 for a spark plug or ignition coil. Professional repair at a shop can range from $150-$500, depending on the part needing replacement and labor rates.
P0312 keeps coming back - why?
If P0312 keeps returning, it means the initial repair did not address the root cause. This often happens if you replaced a part without proper diagnosis. Common reasons for recurrence include a faulty fuel injector, low engine compression, or an intermittent wiring issue that was overlooked.
Will P0312 fail emissions/smog test?
Yes, P0312 will cause your vehicle to fail an emissions or smog test. A misfire directly impacts exhaust emissions, and the active Check Engine Light will prevent the vehicle from passing inspection.
What's the difference between P0312 and P0300?
P0312 specifically indicates a misfire in cylinder 12. P0300, on the other hand, signifies a 'Random/Multiple Cylinder Misfire Detected,' meaning the misfire is not isolated to a single cylinder or is occurring randomly across multiple cylinders.
P0312 on a BMW 7 Series?
P0312 on a BMW 7 Series (specifically V12 models like the 760i/Li) is a common misfire code. These engines are complex, and while spark plugs and ignition coils are frequent culprits, vacuum leaks, fuel injector issues, or even PCV system problems can also contribute to misfires on cylinder 12.
Questions to Ask Your Mechanic
Did you perform a cylinder balance test or relative compression test to confirm the misfire on cylinder 12?
Good answer: Yes, the scan tool confirmed a consistent misfire count on cylinder 12, and the relative compression test showed it was within spec, ruling out a major internal engine issue.
What were the results of the spark plug and ignition coil swap tests for cylinder 12?
Good answer: We swapped the coil from cylinder 12 to cylinder 10, and the misfire followed to cylinder 10, indicating a faulty coil. We also checked the spark plug, and it looked good.
Repair Cost Estimate
DIY Repair
Parts only
$50-$200
Shop Repair
Parts + labor
$150-$500
Save $100-$300 by doing it yourself
Difficulty
intermediate
Time
1-3 hours
Intermediate - requires basic mechanical skills and an OBD-II scan tool for accurate diagnosis. Swapping coils and plugs is manageable for most DIYers.
Cost of Waiting
Continued misfires send unburned fuel into your exhaust system, which can rapidly overheat and destroy your catalytic converter. A $50-$200 spark plug or ignition coil job now prevents a $1,200-$2,500 catalytic converter replacement later.
While You're In There...
If your vehicle uses spark plug wires, they wear out over time and can cause misfires. Replacing them with the spark plugs prevents repeat labor.
These rubber boots protect the coil from moisture and provide insulation. They can crack and degrade, leading to spark loss. Replacing them with new coils or plugs is a smart preventative measure.
Applying dielectric grease to spark plug boots and coil connections helps prevent corrosion, ensures a good electrical connection, and makes future removal easier.
Tools You'll Need
- OBD-II Scan Tool
- Socket Wrench Set
- Spark Plug Socket (specific size for your vehicle)
- Torque Wrench
- Dielectric Grease
- Multimeter (for electrical checks)
- Fuel Pressure Gauge (if suspecting fuel delivery)
- Compression Tester (if suspecting engine compression)
P0312 is a specific misfire code, which often makes diagnosis straightforward. Most repairs involve replacing a relatively inexpensive part, and your engine will run smoothly again.
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Your Next Steps
Step 1: Confirm the misfire is isolated to cylinder 12. Use an OBD-II scan tool to check for pending or active P0312 codes and monitor misfire counts.
Step 2: Perform a 'swap test' with the ignition coil. Move the cylinder 12 coil to another cylinder (e.g., cylinder 11) and see if the misfire code follows. If it does, replace the ignition coil for cylinder 12.
Step 3: If the misfire does not follow the coil, perform a 'swap test' with the spark plug. Move the cylinder 12 spark plug to another cylinder. If the misfire follows, replace the spark plug for cylinder 12. If neither swap fixes it, you'll need to investigate the fuel injector or engine compression.