Ignition

P0312 — Cylinder 12 Misfire Detected

Severity: high Drive with care, to a shop

What P0312 means

The engine control unit monitors crankshaft speed variations and has found that cylinder 12 is not burning its air-fuel mixture properly. This code applies to engines with twelve cylinders. The cause can be a weak spark, a fuel delivery problem, low compression or a wiring fault.

Symptoms

  • Check engine light on or flashing
  • Rough idle
  • Vibration or hesitation when accelerating
  • Loss of power
  • Worse fuel economy
  • Possible stalling
  • Often appears together with P0300

Possible causes

  1. Worn or faulty spark plug in cylinder 12
  2. Failed ignition coil for cylinder 12
  3. Clogged or failed fuel injector
  4. Low compression in the cylinder (valve, rings, gasket)
  5. Damaged wiring or poor connector contact at the coil or injector
  6. Vacuum leak near the cylinder
  7. Software fault or PCM failure (rare)

How to diagnose

  1. Read all codes and freeze frame data to see the speed and load when the misfire occurred
  2. Check per-cylinder misfire counters to confirm cylinder 12 is the problem
  3. Inspect the coil and injector connectors and wiring for corrosion or damage
  4. Pull the cylinder 12 spark plug and check its condition and gap
  5. Swap the coil (and plug) with another cylinder and see if the misfire follows
  6. Test the injector: resistance, clicking and cylinder balance
  7. Look for vacuum leaks at hoses and intake manifold gaskets
  8. Run a compression or leak-down test on cylinder 12

Common fixes

  • Replace the spark plug
  • Replace the cylinder 12 ignition coil
  • Clean or replace the fuel injector
  • Repair the vacuum leak
  • Repair the wiring or connector
  • Do mechanical engine repair (valves, pistons or rings)

Questions and answers

What does P0312 mean?

The engine computer detected a misfire in cylinder 12. Most often the spark plug, coil or injector is at fault, and less often low compression.

Can I drive with P0312?

Only for a short distance under light load. A flashing check engine light means catalytic converter damage is likely, so stop and arrange a tow.

How can I quickly test the coil?

Swap the coil with one from a healthy cylinder, clear the codes and drive. If the misfire code moves to that cylinder, the coil is bad.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.