P0312 – Meaning, Causes, Symptoms, & Fixes

Code P0312 Definition

Cylinder 12 misfire detected.

  • Commonly associated with codes: P0300

What Does P0312 Mean?

P0312 indicates that cylinder number 12 is experiencing misfires. A misfire occurs when an insufficient amount of fuel is burning in a cylinder. The efficient burning of fuel is essential to engine operation as the combustion of fuel is what provides the energy to power the engine. A misfire from one or more cylinders can be caused by many reasons from a faulty ignition system, fuel system or internal engine failure. When P0312 occurs it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine.  Many times P0312 occurs when there are worn out spark plugs, spark plug wires, or a faulty ignition coil.

P0312 Symptoms

  • Check Engine Light is on
  • Check Engine Light flashing
  • Engine runs rough and shaking
  • Lack of power from the engine
  • Fuel smell from the exhaust
  • Hesitations/Jerking when accelerating

P0312 Causes (* = Most Common)

  • Faulty or worn spark plugs *
  • Faulty spark plug wires or coils *
  • Distributor failure *
  • Faulty fuel injector *
  • Vacuum leak
  • Low fuel pressure
  • Camshaft sensor defective
  • Crankshaft sensor defective
  • Engine timing off
  • Leaking head gasket
  • Low engine compression
  • Poor quality fuel

Code P0312 Severity – Severe   

P0312 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.

Code P0312 Common Diagnosis Mistakes

Loose fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.

Code P0312 Diagnosis Steps

Tools Needed to Diagnose:

How To Diagnose P0312

  1. Use FIXD to scan your vehicle to verify P0312 is the only code present. If other codes are present, they must be addressed first.
  2. Check for loose connectors at the ignition coils or for damaged wiring. Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
  3. Check the condition of your spark plugs and spark plug wires. If your vehicle is equipped with individual coil packs instead of spark plug wires, start by removing the cylinder 12 coil and swapping it with the cylinder number 4 coil. If the misfire moved to cylinder number 4, then you have determined that the coil pack is faulty and needs replacement. Replace spark plugs and spark plug wires/ignition coil packs if needed and recheck for misfires.
    1. How to identify a fouled spark plug
    2. How to test spark plug wires
    3. How to gap spark plugs
    4. How to replace spark plugs
    5. How to swap ignition coils
  4. If you have determined that your ignition system is operating correctly, there may be a problem within your fuel system that is causing the random misfires. The following should be checked to ensure the engine is getting the proper amount of fuel:
    1. Check fuel pressure. Low fuel pressure can cause intermittent misfires on multiple cylinders. When the pressure is below the specification, the engine does not receive the proper amount of fuel and will start to lean misfire. The fuel pump or fuel pressure regulator could be the source of the low fuel pressure.
      1. How to check fuel pressure
    2. Check that the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also check that the fuel injector wiring is not damaged and is connected properly.
      1. How to check fuel injectors by ear
      2. How to check fuel injectors with a digital multimeter
  5. If the ignition system and fuel system checks out you may want to perform an engine compression test and leakdown test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
    1. Broken valve spring
    2. Broken piston ring
    3. Worn valve guides
    4. Burned valve
    5. Timing chain or belt skipped tooth and engine is off time.
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