P0303 – Cylinder 3 Misfire Detected

What does P0303 mean?

A misfire has been detected specifically in cylinder 3. The computer monitors crankshaft speed variations to identify which cylinder failed to contribute its power stroke, so this points to a fault isolated to that one cylinder.

What causes P0303?

Listed from most to least common:

1. 1Worn or fouled spark plug in cylinder 3
2. 2Failing ignition coil on cylinder 3
3. 3Clogged or failed fuel injector on cylinder 3
4. 4Faulty spark plug wire
5. 5Vacuum leak affecting cylinder 3
6. 6Low compression in cylinder 3

Code breakdown

Common symptoms

• Check engine light on or flashing
• Rough idle and vibration
• Hesitation and reduced power
• Higher fuel consumption
• Engine may run noticeably unevenly

How to diagnose

1. 1Confirm cylinder 3's physical location for your specific engine layout.
2. 2Inspect the cylinder 3 spark plug for wear, fouling, cracks, or incorrect gap.
3. 3Perform the coil-swap test: move the cylinder 3 coil to a neighboring cylinder and see whether the misfire follows.
4. 4Test or swap the fuel injector.
5. 5Compression test cylinder 3 if ignition and fuel components check out.
6. 6Check for vacuum leaks near that intake runner.

Persistent misfire risks catalytic converter damage. A flashing check engine light means the misfire is severe enough to cause damage quickly — stop driving.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

If misfire codes appear on several cylinders over time and move around, suspect a shared cause such as fuel pressure or a vacuum leak rather than replacing coils one at a time.

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