Ignition

P0305 — Cylinder 5 Misfire Detected

Severity: medium Drive with care, to a shop

What P0305 means

The engine control unit (ECU) watches crankshaft speed and found that cylinder 5 is not burning its air-fuel mixture properly. A cylinder needs spark, the right amount of fuel, enough air and good compression to fire. If any of these is missing, the cylinder misfires and the code is set.

Symptoms

  • Check engine light on or flashing
  • Rough idle and engine shaking
  • Loss of power during acceleration or under load
  • Hesitation or stumbling
  • Worse fuel economy
  • Fuel smell from the exhaust
  • Possible stalling if the misfire is severe

Possible causes

  1. Worn or faulty spark plug in cylinder 5
  2. Failed ignition coil on cylinder 5
  3. Clogged or leaking fuel injector on cylinder 5
  4. Vacuum leak near the cylinder 5 intake runner (intake gasket, cracked hose)
  5. Low compression from valve, piston ring or head gasket problems
  6. Damaged wiring or connector at the coil or injector
  7. Sensor issues affecting the mixture, such as the MAF sensor

How to diagnose

  1. Read all codes and the freeze frame data to see the RPM, load and temperature when the misfire occurred
  2. Check per-cylinder misfire counters to confirm cylinder 5 is the problem
  3. Inspect the wiring and connectors at the cylinder 5 coil and injector
  4. Remove the cylinder 5 spark plug and look for wear, carbon, oil or fuel fouling
  5. Swap the coil (and plug) with a neighboring cylinder, clear codes and see whether the misfire follows
  6. Test the injector by listening, measuring resistance or comparing cylinder balance
  7. Look for vacuum leaks at hoses and the intake gasket, ideally with a smoke machine
  8. If ignition and fuel checks pass, run a compression or leak-down test on cylinder 5

Common fixes

  • Replace spark plugs, preferably as a full set
  • Replace the cylinder 5 ignition coil
  • Clean or replace the cylinder 5 fuel injector
  • Fix vacuum leaks by replacing the intake gasket or damaged hose
  • Repair damaged wiring or the connector
  • Repair the engine mechanically (valves, rings, head gasket) if compression is low

Questions and answers

Can I drive with a P0305 code?

Only briefly and gently if the light is steady. If it flashes, do not drive, since you risk damaging the catalytic converter.

What most often causes P0305?

A worn spark plug or a bad ignition coil on cylinder 5 is the usual culprit. Swapping the coil with another cylinder is a quick way to test it.

Will the code go away by itself?

The light may turn off if the misfire stops, but the cause is still there. The code will come back until the fault is repaired.

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.