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P0312 — Cylinder 12 Misfire Detected

Severe

Quick answer

P0312 means the engine computer has detected that cylinder 12 is misfiring — its air-fuel mixture isn’t burning properly. The most common causes are a worn spark plug or a failing ignition coil on that cylinder. If the check engine light is flashing, stop driving: continued misfires can destroy the catalytic converter.

What it means

Your engine’s computer (the ECM) constantly watches how fast the crankshaft accelerates after each cylinder fires. When cylinder 12 doesn’t contribute the power pulse it should — because the mixture didn’t ignite, ignited weakly, or ignited at the wrong time — the ECM counts it as a misfire. Enough misfires in a short window and it stores P0312 and turns on the check engine light.

The “12” in P0312 refers to the cylinder number, not the firing-order position. Cylinder 12’s location varies by engine — it’s marked in your repair manual, and our VIN tool can point you to the right service info for your exact vehicle.

A misfiring cylinder dumps unburned fuel into the exhaust. The catalytic converter tries to burn that fuel and overheats — which is why a steady light means “fix it soon” but a flashing light means “stop now.”

Common causes

Ordered from most to least likely.

  1. 1.

    Worn or fouled spark plug

    By far the most common cause. Oil or carbon deposits on the tip prevent a clean spark.

  2. 2.

    Failing ignition coil

    Modern engines give each cylinder its own coil. A cracked or shorted coil on cylinder 12 weakens or kills the spark — and coils often fail when hot, so the misfire may come and go.

  3. 3.

    Damaged spark plug wire or boot

    Older engines with plug wires: cracked insulation lets spark leak to ground, especially in damp weather.

  4. 4.

    Clogged or leaking fuel injector

    A dirty injector starves the cylinder; a leaking one floods it.

  5. 5.

    Vacuum leak near cylinder 12

    A torn intake gasket or cracked hose leaning out just that cylinder.

  6. 6.

    Low compression

    Burnt exhaust valve, worn rings, or head gasket — suspect it when the cheap fixes haven’t helped.

  7. 7.

    Wiring or connector damage at the coil/injector

    Chafed harness or corroded pins interrupting power to that cylinder.

How to diagnose it, step by step

Cheapest and most likely checks first.

  1. 1 Scan and read the freeze frame

    Read the codes and note engine temperature, RPM, and load when the misfire happened. If P0312 appears with lean or sensor codes, chase those first.

  2. 2 Inspect spark plug 12

    Pull the plug and read it: oily, sooty, cracked porcelain, or a gap worn past spec all point to the plug. If the set is past its interval, replace all of them.

  3. 3 Swap the ignition coil to another cylinder

    Move cylinder 12’s coil to another cylinder and clear the codes. If the misfire follows the coil — the code changes to that cylinder — the coil is your answer. This free test is the single most useful misfire diagnostic.

  4. 4 Check the injector

    Listen for steady clicking at the injector at idle with a long screwdriver. Silent means no pulse (wiring) or a stuck injector. Injectors can be swap-tested like coils.

  5. 5 Look for vacuum leaks

    Inspect hoses and the intake gasket area near cylinder 12; spray carb cleaner around suspect spots and listen for an idle change.

  6. 6 Run a compression test

    If spark, fuel, and vacuum check out, measure compression on cylinder 12 and compare to its neighbors. More than ~10–15% low points to valves, rings, or head gasket.

Parts & tools you may need

  • OBD-II scanner (code reader with freeze frame / live data)
  • Spark plug socket set with extension
  • Replacement spark plugs (full set, correct part for your engine)
  • Ignition coil (if the swap test confirms it)
  • Torque wrench
  • Carb/brake cleaner (vacuum leak testing)

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Related codes

Frequently asked questions

Can I drive with P0312?
Short distances with a steady check engine light, yes — gently. If the light is flashing, no: raw fuel is overheating the catalytic converter, which costs far more than any likely fix for the misfire.
How much does it cost to fix P0312?
A spark plug is $5–25; an ignition coil typically $40–120. Injector or mechanical causes cost more, but they’re the minority of cases.
Will the code clear itself?
The light can turn off if the misfire stops recurring, but the cause usually doesn’t heal itself. If the code returns after clearing, something real is wrong.
How do I know if it’s the plug or the coil?
Swap the coil to another cylinder and clear the codes. If the misfire code moves with the coil, it’s the coil; if it stays, suspect the plug, injector, or compression.