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?

Only if the light is steady and the car is running smoothly. If the light returns flashing, stop immediately.

Codes › P0303

P0303: Cylinder 3 is misfiring

Cylinder 3 Misfire Detected

High severityCan you keep driving?

If the light is flashing, stop and do not drive. Raw fuel in the exhaust will melt the catalytic converter within minutes. A steady light with noticeable shaking means get to a shop the same day.

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 it costs

Spark plug set$80 to $250
Ignition coil (one)$150 to $400 installed
Fuel injector (one)$150 to $400 installed
Compression or valve repair$900 to $3,500

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 tracks per-cylinder misfire counts live so you can watch the rate on cylinder 3 and see whether it is constant or only happens at certain RPMs or load levels. Download Lewis.

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?: Only if the light is steady and the car is running smoothly. If the light returns flashing, stop immediately.