P0303 Code: Cylinder 3 Misfire Detected

🟠 Moderate severity · drivable

You can usually keep driving short distances, but repair it soon to protect the catalytic converter — and stop right away if the light flashes.

What P0303 means

The computer has flagged cylinder 3 for misfiring. As a single-cylinder code, P0303 tells you the fault is localized to cylinder 3 — its spark plug, ignition coil, or injector — rather than a system-wide issue. That localization is what separates P0303 from P0300, the random-misfire code that points to shared causes like fuel delivery or air leaks.

Symptoms

Check engine light on (flashing if severe)
Rough, shaky idle
Loss of acceleration and power
Reduced fuel mileage

Common causes

most common: Worn or fouled spark plug on cylinder 3
common: Failing ignition coil or plug wire for cylinder 3
common: Clogged or leaking fuel injector on cylinder 3
less common: Low compression in cylinder 3 (rings, valve, or gasket)

How to fix it

Fix	DIY?	Typical cost
Swap cylinder 3's spark plug and coil with another cylinder to confirm whether the fault follows the parts	Yes	$0–20
Replace the cylinder 3 spark plug	Yes	$10–50
Replace the cylinder 3 ignition coil	Yes	$50–200
Have cylinder 3 compression-tested	Mechanic	$100–200

Frequently asked questions

Does P0303 point only to cylinder 3?

Yes — a single-cylinder misfire almost always means the problem is on that cylinder's plug, coil, or injector. A misfire that hops between cylinders typically sets P0300 instead.

How do I isolate a P0303 quickly?

Swap the cylinder 3 coil and plug with another cylinder. Re-scan: if the misfire moves, the bad part moved with it. If it stays on 3, check the injector and compression next.

Is it safe to drive with a P0303?

Briefly, with a steady light. A misfire wastes fuel into the catalytic converter, so get it fixed — and if the light flashes, stop driving immediately.