P0312Generic code

P0312 Code: Cylinder 12 Misfire Detected

The P0312 code means the computer detected failed combustion events in cylinder 12. The most common causes are the spark plug, the ignition coil or the fuel injector on that cylinder, and less often a compression problem. You can drive with caution if the light is steady, but a flashing light means you should slow down and get it fixed.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$20 to $600
Difficulty
Medium
System
Ignition and Misfires
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0312 code means

The computer uses the crankshaft position (CKP) sensor to measure the small burst of acceleration that each combustion event produces. When a cylinder does not deliver it, the computer counts a misfire. If the number of misfires goes over the limit in a block of engine revolutions, it stores the code for the cylinder involved. That is how the P0312 code is set.

Having a code for cylinder 12 specifically is a big advantage: the problem is almost always in something that belongs to that one cylinder. Cylinders 7 to 12 only exist on engines with eight, ten or twelve cylinders, almost always in a V layout. On those engines, reaching the spark plugs and coils on the rear bank can be awkward, so it pays to diagnose carefully before you take anything apart.

If the check engine light is flashing, the rate of misfires can damage the catalytic converter. Slow down and avoid heavy loads until it is repaired. Cylinder numbering is set by the manufacturer. On most inline engines, cylinder 1 is the one closest to the timing belt or chain, and on V engines the odd and even cylinders are usually on different banks. Confirm it in the service manual before you start.

See the complete guide to P0300 misfire codes for the swap method of diagnosis.

P0312 code symptoms

  • Engine shaking, especially at idle or under load.
  • Check engine light steady or flashing.
  • Loss of power.
  • Smell of unburned fuel from the exhaust.
  • Higher fuel use.
  • Hesitation or jerking when you accelerate.

Likely causes of the P0312 code

  1. Worn or damaged spark plug in cylinder 12. The cheapest and most frequent cause.
  2. Cylinder 12 ignition coil with high voltage leaks or a damaged internal winding.
  3. Deteriorated spark plug wire or boot on engines that use them.
  4. Cylinder 12 injector clogged or not pulsing. The cylinder is not getting enough fuel.
  5. Vacuum leak near the cylinder. The intake manifold gasket on that side.
  6. Low compression from valves, piston rings or the head gasket.
  7. Damaged camshaft or rocker arm on high-mileage engines.

How to diagnose the P0312 code step by step

  1. Confirm the cylinder with the misfire counters on the scan tool, at idle and under load.
  2. Inspect the cylinder 12 spark plug. Color, electrode wear and gap.
  3. Swap the cylinder 12 coil with the one from a neighboring cylinder, clear the codes and drive. If the misfire moves to the other cylinder, the coil is the culprit.
  4. Do the same with the spark plug if the misfire did not follow the coil.
  5. Check injector 12. Resistance, pulse with a test light, and the click with a stethoscope.
  6. Measure the compression of cylinder 12 and compare it with the others (the difference should be under 10 to 15 percent).
  7. If the compression is low, run a leak-down test to find out whether it is a valve, rings or the head gasket.

How to fix the P0312 code

  • Replace the spark plug with the correct type and gap (on a V8 it is best to change them all at once).
  • Replace the damaged coil or plug wire.
  • Clean or replace the injector.
  • Repair the vacuum leak or the mechanical cause.
  • Clear the codes and check that the misfire counters stay at zero.

Technical reference data

  • Typical coil primary resistance: 0.4 to 1.5 ohms.
  • Typical spark plug gap: 0.8 to 1.1 mm (about 0.031 to 0.043 in).
  • Typical compression on a gasoline engine: 10 to 14 bar (about 145 to 200 psi).
  • These values are for orientation only. Check the manufacturer data for your engine.

Brand-specific notes

Rear bank spark plugs and coils are the hardest to reach on a V engine, so confirm the failing part before you disassemble anything.

P0312 on GM

On GM V8 engines, cylinders 7 and 8 are usually at the back of each bank, near the firewall, where heat wears out the coils and boots sooner.

P0312 on Ford

On Ford V8 engines, cylinders 7 and 8 are usually at the back of each bank, near the firewall, where heat wears out the coils and boots sooner. On the Ford 5.4 three-valve, the spark plugs can break when you remove them, so follow the manufacturer procedure.

Frequently asked questions about P0312

Can I drive with a P0312 code?
With caution if the light is steady. If it is flashing, slow down and take the vehicle to a shop, because the catalytic converter can be damaged.
Where is cylinder 12?
Cylinder numbering is set by the manufacturer. On most inline engines, cylinder 1 is the one closest to the timing belt or chain. On V engines, the odd and even cylinders are usually on different banks. Confirm it in the service manual before you take anything apart.
Do I have to replace all the spark plugs?
If they have the same wear, yes. One new plug next to worn ones leaves the engine unbalanced.
How much does it cost to fix P0312?
Between $20 and $600 if it is a spark plug, a coil or an injector. It costs more if there is a mechanical problem.
What does the P0312 code mean?
It is a generic OBD2 code that means the computer detected misfires in cylinder 12. The cause is usually in the spark plug, coil or injector for that cylinder.
What is the most common cause of P0312?
The P0312 code most often comes from a worn spark plug or a weak ignition coil on cylinder 12. Swapping the coil with a neighboring cylinder is the quickest way to tell.

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