Ignition

P0308 — Cylinder 8 Misfire Detected

Severity: medium Drive with care, to a shop

What P0308 means

The engine control unit monitors crankshaft speed and detected that cylinder 8 is not burning its air-fuel mixture properly. Combustion needs spark, fuel, air and good compression, and a problem with any of them can cause a misfire. Because the code points to one cylinder, the cause is usually something specific to that cylinder.

Symptoms

  • Rough or shaky idle
  • Hesitation or jerking when accelerating
  • Check engine light on or flashing
  • Loss of power, especially under load
  • Poor fuel economy
  • Fuel smell from the exhaust

Possible causes

  1. Worn or faulty spark plug in cylinder 8
  2. Failed ignition coil on cylinder 8
  3. Loose or damaged coil connector or boot
  4. Clogged or leaking fuel injector on cylinder 8
  5. Vacuum leak near that cylinder's intake runner (intake gasket, cracked hose)
  6. Low compression from a valve, piston rings or head gasket

How to diagnose

  1. Scan for codes and see whether other misfire or fuel trim codes are stored
  2. Review freeze frame data and per-cylinder misfire counts
  3. Pull the cylinder 8 spark plug and inspect it for wear, deposits, oil or a cracked insulator
  4. Inspect the coil and injector connectors and wiring
  5. Swap the coil (and plug if needed) with another cylinder and see whether the misfire follows
  6. Test the injector: operation, resistance and cylinder balance
  7. Check the intake for vacuum leaks, for example with a smoke machine
  8. If the electrical checks pass, run a compression test and a leak-down test if needed

Common fixes

  • Replace the spark plugs, ideally as a full set
  • Replace the cylinder 8 ignition coil
  • Clean or replace the fuel injector
  • Repair vacuum leaks: intake gasket or damaged hose
  • Repair the engine if compression is low (valves, rings, head gasket)

Questions and answers

What does P0308 mean?

The engine computer detected a misfire in cylinder 8, meaning the mixture there is burning poorly or not at all. The most common culprits are the spark plug, ignition coil or fuel injector for that cylinder.

Can I drive with a P0308 code?

Only carefully and for a short distance, avoiding heavy loads. If the check engine light is flashing, stop, because unburned fuel can damage the catalytic converter.

How do I find the cause of P0308 quickly?

Swap the cylinder 8 coil with one from another cylinder and clear the code. If the misfire moves, the coil is bad. If not, check the plug, injector and compression.

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.