P0303: Cylinder 3 Misfire Detected

Common causes

• A worn or fouled spark plug
• A failing ignition coil or plug wire
• A clogged or failing fuel injector
• A vacuum leak near cylinder 3
• Low compression from a valve or head gasket problem (less common)

Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.

Symptoms

• Rough idle or shaking
• Hesitation or loss of power
• A flashing check engine light when the misfire is severe

Is it safe to drive?

If the check engine light is flashing, stop driving and have it towed or repaired where it sits; unburned fuel can overheat and ruin the catalytic converter. With a steady light, it's usually OK to drive gently to a shop, but fix it soon.

How to check it with a scanner

1. Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
2. With the engine off, swap the cylinder 3 coil (or plug wire) with a neighboring cylinder, clear the code and see if the misfire moves.
3. Pull and inspect the cylinder 3 spark plug for wear, oil or fouling.
4. If spark is good, have the injector checked and a compression test done on that cylinder.
5. After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.

A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.

Questions

Related codes

• P0300: Random or multiple cylinder misfire
• P0304: Misfire in cylinder 4
• P0301: Misfire in cylinder 1
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