P0303 P0xxx · Powertrain (P) High severity
Cylinder 3 Misfire Detected
Cylinder 3 misfire. Same diagnostic approach as other cylinder-specific misfire codes: confirm the cylinder, then swap-test to move the fault.
Severity guidance: High — stop driving and diagnose now; risk of further damage
Symptoms you will notice
- Rough idle
- Vibration under load
- Flashing MIL
- Catalyst-damaging raw fuel
Common causes
Ordered from most to least likely — start at the top.
- 1 Failed coil or plug
- 2 Faulty injector
- 3 Vacuum leak at the cylinder 3 runner
- 4 Low compression
How to diagnose it
Cheapest and least invasive first — most of these codes are not a parts problem.
- ✓ Read per-cylinder misfire counters at idle and under load.
- ✓ Swap ignition components between cylinders 3 and 4.
- ✓ Smoke-test the intake manifold if the runner gasket is suspect.
- ✓ Compression and leak-down test if hardware is still suspect.
Repair options
- Replace the coil or plug
- Service the injector
- Repair the intake runner leak
- Mechanical repair if compression is low
P0303 — frequently asked questions
Does the cylinder number follow the firing order? +
No — cylinder numbers are physical positions. Check the manual for the layout of your engine.
Can I keep driving briefly? +
A light misfire at idle only can wait a short time. A misfire under load is a stop-now condition.
Confirm the diagnosis before you buy parts
Send us the code, freeze-frame data and the vehicle — our engineers will tell you what to test first.