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P0303: Cylinder 3 Misfire Detected

What this check-engine code means in plain English, what usually causes it, and what a fix typically costs.

Severity: Urgent — can cause catalytic converter damage if driven extensively

What P0303 means

The engine control module has detected that cylinder 3 is not firing properly or is misfiring. This means the air-fuel mixture in that cylinder isn't combusting correctly, causing rough running and potentially sending unburned fuel into the exhaust system where it can overheat and damage the catalytic converter.

What causes P0303?

Likely
Faulty spark plug or ignition coilWorn spark plugs or a failing ignition coil on cylinder 3 are the most common causes of single-cylinder misfires.
Likely
Fuel injector problemA clogged or malfunctioning fuel injector for cylinder 3 can prevent proper fuel delivery and cause misfires.
Possible
Vacuum leak or intake gasketA vacuum leak near cylinder 3 or a leaking intake manifold gasket can create a lean condition causing misfires.
Less common
Low compression or mechanical issueWorn piston rings, damaged valves, or a blown head gasket can cause low compression in cylinder 3 leading to misfires.

Is P0303 safe to drive?

Drive it?
Short distances only, avoid extended driving
Watch for
Rough idle, loss of power, strong fuel smell, or flashing check engine light
DIY-friendly?
Spark plug replacement is DIY-friendly; coil swapping to test requires moderate skill

You can drive short distances to a repair shop, but avoid extended driving or highway speeds. Continued misfires send unburned fuel into the catalytic converter, which can overheat and cause expensive damage. If the check engine light is flashing, stop driving immediately as this indicates severe misfires.

P0303 repair cost

~$100 – $1000+

Industry-average estimates. Cost depends on the root cause, from simple spark plug replacement to more complex repairs like injectors, coils, or internal engine work.

Which one is it for your car?

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