P0303: Cylinder 3 Misfire Detected
P0303 means Cylinder 3 Misfire Detected. The engine computer has detected misfires in cylinder 3. It detects them by measuring small changes in crankshaft speed.
Can I keep driving? Limit driving and diagnose promptly. Continued driving can cause damage, stalling or limp mode.
Common symptoms
- Check engine light on, or flashing during misfire
- Rough idle and shaking
- Hesitation and loss of power
- Poor fuel economy
Possible causes
- Worn or fouled spark plug in cylinder 3
- Faulty ignition coil or plug wire for cylinder 3
- Clogged or failed fuel injector for cylinder 3
- Vacuum leak near cylinder 3's intake port
- Low compression in cylinder 3 (valve, rings or head gasket)
How to diagnose P0303
- Swap the ignition coil from cylinder 3 with a neighbouring cylinder, clear codes and see whether the misfire follows the coil.
- Do the same with the spark plug.
- Check injector operation and resistance for cylinder 3.
- Do a compression test on cylinder 3 if ignition and fuel are fine.
- Check for coolant or oil on the spark plug, which points to a gasket problem.
A flashing check engine light means misfires severe enough to overheat and damage the catalytic converter. Reduce load and get it checked right away.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.