P0303 — Cylinder 3 Misfire Detected

What it means

The engine control module has detected misfire in cylinder 3, meaning combustion in that cylinder failed or was incomplete. The module identifies it from instantaneous crankshaft speed variations read through the crankshaft position sensor. Typical symptoms are a shaking idle, loss of power and higher fuel consumption; a flashing check engine lamp means raw fuel is reaching the catalytic converter and destroying it.

Likely causes (most common first)

1. Worn, fouled or wet spark plug in cylinder 3
2. Failed ignition coil or high voltage lead on that cylinder
3. Clogged, leaking or electrically open fuel injector
4. Vacuum leak at the intake manifold gasket or a cracked hose near the cylinder
5. Low compression: burnt valve, worn piston rings, blown head gasket
6. Broken wiring or a failed ECM output driver for the coil or injector

Can you drive with it?

Short low load trips are possible, but with a flashing lamp stop driving at once: the catalytic converter is being destroyed.