P0312: Cylinder 12 Misfire Detected

What it means

Misfires concentrated on cylinder 12. A cylinder-specific code points away from shared causes (fuel pressure, MAF) toward components dedicated to that cylinder — coil, spark plug, injector, compression.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The engine controller looks for crankshaft speed changes that indicate a cylinder did not produce its expected power. P0312 means the detected misfire is attributed to cylinder 12, so checks should focus on that cylinder’s spark, injector, wiring, and mechanical condition.

Common causes

Most likely first.

1. Faulty spark plug or ignition coil for cylinder 12
2. Clogged or faulty cylinder 12 fuel injector
3. Damaged wiring or connector at the cylinder 12 ignition or injector
4. Low compression in cylinder 12

Symptoms

• Check engine light on or flashing
• Rough idle and uneven running
• Power loss, hesitation under acceleration
• Increased fuel consumption
• Possible smell of unburned fuel from exhaust

How to diagnose it

1. Review freeze-frame data and cylinder misfire counts to confirm cylinder 12 is implicated and identify the conditions when it occurred.
2. Inspect cylinder 12’s spark plug, coil, injector connector, and wiring for wear, poor seating, corrosion, or damage.
3. Where coils are interchangeable, move the cylinder 12 coil to another cylinder and check whether the misfire count moves with it.
4. Test the cylinder 12 spark plug and injector operation, comparing them with other cylinders according to the vehicle’s service information.
5. If the fault remains on cylinder 12, measure compression or perform a leak-down test and compare with the manufacturer’s specification.

Common fixes

• Secure or repair a loose or damaged cylinder 12 ignition or injector connection.
• Replace a worn or fouled cylinder 12 spark plug.
• Replace the cylinder 12 coil if testing shows the misfire follows that coil.
• Clean or replace the cylinder 12 injector if its operation is faulty.
• Repair the cause if cylinder 12 compression is low.

Can I keep driving?

A flashing check engine light means the misfire may damage the catalytic converter, so stop as soon as it is safe. If the light is steady and the engine runs smoothly enough, limit driving and avoid hard acceleration until the fault is diagnosed.