P0303: Cylinder 3 is misfiring
Cylinder 3 Misfire Detected
Cylinder 3 is misfiring. If the light is flashing, stop driving: raw fuel in the exhaust can melt the catalytic converter within minutes. If the light is steady, get it checked the same day.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
What it means
P0303 means the crankshaft position sensor detected a speed drop during the cylinder 3 power stroke, which is how the PCM measures misfires. A misfire on cylinder 3 is a complete or partial failure to ignite the air-fuel mixture in that cylinder. The unburned mixture passes into the exhaust and can ignite inside the catalytic converter, which is designed to handle trace gases, not large fuel loads. Repeated misfires do not just waste power; they cook converters and can leave a deposit of burnt oil or fuel on the converter substrate.
Common causes
- Faulty ignition coil on cylinder 3Swap the coil to confirm. If the misfire follows it, replace the coil.
- Worn spark plug on cylinder 3A plug past 60,000 to 100,000 miles is the first thing to replace if you have not done plugs recently.
- Cylinder 3 injector faultA clogged or dead injector prevents proper fuel delivery. Check for P0203 alongside this code.
- Low compression on cylinder 3A failed valve seat or worn rings means the cylinder cannot build the pressure needed for ignition.
- Vacuum leak near cylinder 3 intake runnerExcess air leans the mixture to a misfire threshold, especially at idle.
What the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- Spark plug set
- Ignition coil (one)
- Fuel injector (one)
- Compression or valve repair
Symptoms
- Check engine light flashing or steady
- Engine shake and vibration at idle
- Stumble or flat spot when accelerating
- Reduced power and fuel economy
- Possible raw fuel smell from exhaust
What to do next
Swap the cylinder 3 coil with a known good cylinder. Check if the code moves. If the coil is fine, inspect the spark plug. A plug gap out of spec or an electrode worn flat needs replacement. If both check out, have the injector tested for balance rate and run a compression test to rule out mechanical failure.
Lewis reads P0303 alongside any other misfire codes and shows live RPM and engine load, so you can see whether cylinder 3 is the only one flagged. Get Lewis on the App Store or Google Play.
Related codes
FAQ
- Cylinder 3 fires fine at high RPM but misfires at idle. Why?
- Low-speed misfires often point to a lean mixture from a vacuum leak or a coil that loses voltage at low load. High-RPM misfires with a healthy idle suggest ignition or fuel delivery failure at load.
- How quickly can a misfire destroy a catalytic converter?
- A severe misfire with a flashing light can overheat a converter in as little as a few minutes of driving. A slow, intermittent misfire may take weeks.
- Is it safe to clear the code and monitor it?
- Clearing the code does not fix the misfire, and it erases information a shop can use. Get it checked today. If the light comes back flashing, stop driving.
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