OBD codes / P0303
P0303
SeriousCylinder 3 Misfire Detected
Treat P0303 with some urgency. A steady check-engine light with a mild misfire is usually drivable short-term, but a FLASHING light means raw fuel is reaching the catalytic converter and can ruin it — pull over and have it towed or diagnosed. Misfires also tend to worsen, so don't put the fix off.
What this code means
A misfire means a cylinder isn't burning its air-fuel mixture properly. P0303 points to one specific cylinder — number 3 — which makes it easier to diagnose than a random misfire. The usual cause is that cylinder's spark plug or ignition coil; a quick test is to swap the coil or plug with a neighboring cylinder and see if the misfire follows. Less commonly it's a clogged or dead fuel injector, or low compression in that cylinder from a valve or ring problem. Because unburned fuel can overheat the catalytic converter, a steady misfire shouldn't be left for long.
Symptoms you might notice
- Rough idle or shaking, often felt at a stop
- Loss of power or hesitation under acceleration
- Flashing check-engine light during an active misfire
- Reduced fuel economy
- Engine may stumble or be hard to start
What it costs to fix
Typical range: $80–$600 · about ~0.3–3 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Spark plug (single) | $4–$30 | ~0.3–1.5 hrs |
| Ignition coil (single) | $30–$150 | ~0.3–1 hrs |
| Fuel injector (single) | $100–$400 | ~1–3 hrs |
A single plug or coil is the cheap, common fix; an injector is pricier; low compression (a valve or ring issue) is a major repair that varies too much to quote — get it diagnosed if plug, coil, and injector all check out.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0303 for my exact vehicle →
How to fix P0303
Easy — basic tools, most owners can do this
- Verify cylinder 3's position
Confirm from your engine's cylinder diagram which one is number 3 — on many V6s it's on a different bank than you'd guess. Getting this right is the foundation of every test that follows.
- Swap coil 3 to another cylinder and watch the code
Put cylinder 3's coil on a different cylinder, clear the code, and drive. The misfire moving with the coil (a new P030x code) convicts the coil; the misfire staying on cylinder 3 clears it.
- Inspect plug 3 for fouling or wear
Remove the cylinder 3 spark plug and read it — oil on the threads, a chalky white or soot-black tip, or a worn gap all explain a misfire. Replace it if it's questionable; on high-mileage plugs, do the whole set.
- Check the cylinder 3 injector
Listen for the injector's click at idle or swap it with a neighbor to see whether the code follows. If the misfire travels with the injector, replace that injector.
- Compression-test cylinder 3 if everything else passes
A stubborn P0303 with proven spark and fuel points to the cylinder itself. A compression reading well below its siblings means valve or ring trouble that a DIY parts swap won't fix.
DIY or shop? Everything through the injector swap is realistic DIY on most engines — the parts are cheap and the tests are simple eliminations. Go to a shop if compression tests low, if the misfire is intermittent and hard to catch, or if reaching cylinder 3 requires removing the intake manifold on your engine.
Tools for the DIY steps: OBD-II scanner · Spark plug socket · Ratchet with extensions · Spark plug gap gauge · Compression tester (optional)
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0303.
Frequently asked
Can I drive with P0303?
With a steady (not flashing) light and only a mild misfire, short trips are usually fine — but get it looked at soon. If the light is flashing, the active misfire is dumping raw fuel into the catalytic converter and can destroy it; stop driving and have the car towed.
How do I know if it's the coil or the spark plug?
Swap cylinder 3's coil (or plug) with a neighboring cylinder and clear the code. If the misfire moves to the other cylinder, the part you moved is the culprit — a cheap way to avoid replacing the wrong thing.
How much does it cost to fix P0303?
Often modest — a single spark plug or coil is the common, inexpensive fix. It rises if the cause is a fuel injector, and a compression problem (valve or ring) is a major repair that varies widely. Confirm the cause before spending.
Should I replace all the coils or just cylinder 3?
If only cylinder 3 is misfiring, replacing just its coil/plug is reasonable. Some owners replace all of them at once if the parts are old and high-mileage, to avoid chasing the next one — but it isn't required to fix P0303.
Seeing P0303 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0303 for my vehicleSymptoms that trigger P0303
Drivers who get this code often notice one of these — each has its own likely-causes and cost guide:
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0303 from your phone — see our honest guide to the best OBD2 Bluetooth adapters.
Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.