P0303: Cylinder 3 Misfire Detected

What it means

Misfires concentrated on cylinder 3. A cylinder-specific code points away from shared causes (fuel pressure, MAF) toward components dedicated to that cylinder — coil, spark plug, injector, compression. Verify by swapping the suspected coil/plug to a different cylinder; if the code follows, the part is bad.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The engine control module detects misfires by monitoring changes in crankshaft speed as each cylinder contributes power. This code identifies repeated misfire activity associated with cylinder 3, directing checks toward that cylinder's ignition, fuel delivery, and compression.

Common causes

Most likely first.

1. Ignition coil for cylinder 3 failed
2. Spark plug for cylinder 3 worn or fouled
3. Cylinder 3 injector clogged

Symptoms

• Check engine light on or flashing
• Rough idle and uneven running
• Power loss, hesitation under acceleration
• Increased fuel consumption
• Possible smell of unburned fuel from exhaust

How to diagnose it

1. Read freeze-frame data and cylinder misfire counts to see whether cylinder 3 misfires at idle, under load, or in a particular temperature range.
2. Inspect cylinder 3's spark plug, coil, connector, and wiring for wear, fouling, cracks, oil, or a poor electrical connection.
3. If the engine uses interchangeable coils, swap the cylinder 3 coil with another cylinder and see whether the misfire code or count follows the coil.
4. If the misfire stays on cylinder 3, inspect or test its spark plug and injector, checking injector command and operation as appropriate.
5. If ignition and fuel checks do not explain the fault, perform a compression or leak-down test on cylinder 3 and compare results with the manufacturer's specification.

Common fixes

• Repair a loose or damaged connection at cylinder 3's coil or injector.
• Replace a worn or fouled cylinder 3 spark plug.
• Replace the cylinder 3 ignition coil if the misfire follows it during a swap test.
• Clean or replace a faulty cylinder 3 injector when testing confirms poor delivery.
• Repair an engine mechanical problem identified by compression or leak-down testing.

Can I keep driving?

If the check-engine light is flashing, stop driving as soon as it is safe; an active cylinder 3 misfire can overheat and damage the catalytic converter. With a steady light and mild symptoms, drive only gently for a short distance and avoid acceleration under load.