P0312
UniversalThe Powertrain Control Module (PCM) has detected that cylinder 12 is not contributing to engine power output as expected.
Medium SeverityDTC P0312 indicates that the engine's computer has detected an intermittent or continuous misfire specifically in cylinder 12. This means that cylinder 12 is not firing correctly, leading to reduced engine power, rough idle, and potentially damage to the catalytic converter over time.
⚠️ Before You Begin (Safety First)
Always allow the engine to cool down before working on it. Disconnect the negative battery terminal to prevent accidental electrical shocks or short circuits. Wear appropriate personal protective equipment, including gloves and eye protection. Be cautious when handling hot engine components and high-voltage ignition systems.
Common Symptoms
- Check Engine Light illuminated or flashing
- Rough idle
- Engine hesitation or stumble
- Reduced engine power
- Poor fuel economy
- Engine vibrations
- Smell of raw fuel from the exhaust
Common Causes
- Faulty spark plug in cylinder 12
- Defective ignition coil for cylinder 12
- Clogged or faulty fuel injector for cylinder 12
- Low compression in cylinder 12 (due to worn piston rings, valve issues, or head gasket problems)
- Vacuum leaks affecting cylinder 12
- Wiring issues to the ignition coil or fuel injector for cylinder 12
- Faulty PCM (less common)
Frequently reported on: Mercedes-Benz S600 (2000-2017), BMW 760Li (2003-2016), Audi A8 W12 (2005-2018), Aston Martin DB9 (2004-2016), Rolls-Royce Ghost (2009-2020)
Diagnostic Steps
- Visually inspect the ignition coil and spark plug wire (if applicable) for cylinder 12 for any damage.
- Remove and inspect the spark plug from cylinder 12. Look for signs of fouling, damage, or improper gap.
- Swap the ignition coil from cylinder 12 with a known good coil from another cylinder (e.g., cylinder 11) and clear the code. If the misfire moves to the new cylinder, the coil is faulty.
- If the coil swap doesn't help, swap the fuel injector from cylinder 12 with one from another cylinder. If the misfire moves, the injector is faulty.
- Check for vacuum leaks near cylinder 12 using a smoke machine or by listening for hissing sounds.
- Perform a compression test on cylinder 12 to check for internal engine issues.
👨🔧 When to Call a Professional
If basic troubleshooting (spark plug, ignition coil, injector swap) does not resolve the issue, or if you detect symptoms of internal engine damage like compression loss, it's best to consult a certified mechanic. Complex diagnostics involving fuel pressure, camshaft timing, or internal engine components often require specialized tools and expertise.
Commonly Asked Questions
What does P0312 specifically mean?
P0312 indicates that your vehicle's engine control unit has detected a misfire occurring specifically in cylinder number 12, meaning it's not firing correctly.
Is it safe to drive with a P0312 code?
Driving with a misfire can damage your catalytic converter over time and reduce engine performance. It's recommended to diagnose and fix the issue promptly.
How do I locate cylinder 12?
Cylinder numbering varies by manufacturer. Consult your vehicle's service manual or an online resource specific to your car's make and model to correctly identify cylinder 12.
What are the most common causes of a P0312 misfire?
The most frequent causes include a faulty spark plug, a defective ignition coil, or a clogged/failed fuel injector specific to cylinder 12.
After replacing the part, how do I clear the code?
You can clear the code using an OBD-II scan tool. Disconnecting the battery for 15 minutes can also clear codes, but will reset other learned engine parameters.