Ignition

P0303 — Cylinder 3 Misfire Detected

Severity: medium Drive with care, to a shop

What P0303 means

The engine control unit watches crankshaft speed and noticed that cylinder 3 is not burning its air-fuel mixture properly. Each cylinder needs three things to fire cleanly: a strong spark, the correct amount of fuel and good compression. If any one is missing in cylinder 3, the ECU sets P0303.

Symptoms

  • Rough, shaky idle
  • Check Engine light on or flashing
  • Loss of power and hesitation when accelerating
  • Worse fuel economy
  • Fuel smell from the exhaust
  • Stumbling or vibration under load

Possible causes

  1. Worn or fouled spark plug in cylinder 3
  2. Failed ignition coil or damaged plug wire on cylinder 3
  3. Clogged or faulty fuel injector on cylinder 3
  4. Vacuum leak near cylinder 3's intake runner (intake gasket, cracked hose)
  5. Low compression from a valve, piston rings or head gasket
  6. Less commonly, a bad camshaft or crankshaft position sensor or damaged wiring

How to diagnose

  1. Read the codes and freeze frame data to see the RPM and load when the misfire occurred and whether other codes are stored
  2. Check per-cylinder misfire counters if your scan tool shows them
  3. Pull the cylinder 3 spark plug and inspect it for wear, oil, carbon or a cracked insulator
  4. Swap the cylinder 3 coil (and plug) with a neighbor and see if the misfire follows
  5. Inspect the coil and injector connectors and wiring, and measure injector resistance if needed
  6. Look for vacuum leaks around the intake manifold and hoses, using a smoke machine if possible
  7. Test injector operation and cylinder balance
  8. If ignition and fuel check out, run a compression test and a leak-down test on cylinder 3

Common fixes

  • Replace the spark plugs, ideally as a full set
  • Replace the cylinder 3 ignition coil
  • Clean or replace the cylinder 3 fuel injector
  • Fix the vacuum leak by replacing the gasket or damaged hose
  • Repair internal engine damage (valves, rings, head gasket) based on compression results

Questions and answers

Can I drive with a P0303 code?

Briefly and gently to get to a shop, yes. If the check engine light is flashing, stop and avoid driving further to protect the catalytic converter.

What most often causes P0303?

A worn spark plug or failed ignition coil on cylinder 3 is the most common cause. Swapping the coil with a neighboring cylinder is a quick way to see if the misfire moves.

Can I just clear the P0303 code?

Clearing it only turns the light off for a while. Unless the cause is fixed, the misfire will return and the code will come back, and continued misfiring can damage the catalytic converter.

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.