P0303: Cylinder 3 Misfire Detected
Cylinder 3 is not firing reliably, so the engine computer has logged a misfire on that cylinder.
What P0303 means
P0303 tells you the misfire monitor has isolated cylinder 3 as the one losing power strokes. The computer uses the crank sensor signal and the firing order to know which cylinder was due to fire when the speed dropped. Most of the time the fix is ordinary: a coil, a plug or an injector for that cylinder. But a cylinder in the middle of the engine, like number 3 on many four cylinder engines, is also where a head gasket leak can show up, because it sits next to coolant passages. Signs include a misfire that is worst right after a cold start, white sweet smelling exhaust, and coolant that keeps going low. A shop separates these causes with a swap test, then a compression and leak down test, and sometimes a combustion gas test on the coolant. A simple ignition fix is cheap. Letting a head gasket problem go can overheat the engine, so get it looked at soon.
Common symptoms
- Check engine light on, or flashing when the misfire is bad
- Rough or shaky idle
- Hesitation or jerking when you accelerate
- Loss of power, especially going uphill or merging
- Worse gas mileage
Most common causes
- Failed ignition coil or plug wire on cylinder 3
- Worn spark plug
- Clogged or dead fuel injector
- Head gasket leak between cylinder 3 and a coolant passage
- Burnt or tight valve
- Vacuum leak at the intake runner
What to check yourself first
Do a coil swap with a neighboring cylinder and see whether the code moves. Check the coolant level when cold; a dropping level plus a cylinder 3 misfire, especially on a cold start with white exhaust smoke, can point to a head gasket and needs a shop test.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace ignition coil | $238-$320 | Swap test first; replace only the coil that carries the misfire with it. |
| Replace spark plugs | $288-$399 | If one plug looks white or wet with coolant, tell the shop before they replace parts. |
| Replace fuel injector | $768-$953 | Done after the injector fails an electrical or flow test. |
| Check engine light diagnosis at a shop | $125-$183 | Includes a cylinder leak down test to check for a head gasket or valve issue. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0300 Random/Multiple Cylinder Misfire Detected
- P0302 Cylinder 2 Misfire Detected
- P0304 Cylinder 4 Misfire Detected
- P0203 Injector Circuit/Open - Cylinder 3
- P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)