P0303

What P0303 means

The engine computer watches how smoothly each cylinder fires by tracking tiny changes in crankshaft speed. Cylinder 3 is firing late, weak, or not at all, often enough that the computer flagged it. It is not telling you which part caused it, just which cylinder is misbehaving.

Is it safe to drive

Drive it as little as possible until this gets looked at. A misfire dumps unburned fuel into the exhaust, and that fuel can overheat and destroy the catalytic converter, which is a much more expensive repair than whatever caused the misfire. If the check engine light is flashing rather than steady, that is the computer telling you the misfire is severe enough to cause damage right now. In that case, stop driving and get it towed in. If it is a steady light and the car otherwise drives normally, you can usually get it to us safely, but don't put off the appointment. Watch for rough idle, hesitation on acceleration, a smell of raw fuel, or the light starting to flash.

Common causes, most likely first

Spark plugs, especially if they haven't been changed in a while, are the most common cause. Next most common is an ignition coil, or the wiring going to it, on that cylinder — some engines have one coil per cylinder, others have one coil firing multiple cylinders through plug wires, and that changes what we check. After that: a fuel injector that's clogged or failing on cylinder 3, a vacuum leak affecting that cylinder specifically, or low compression from a worn cylinder, weak valve, or bad head gasket. Which of these is likely depends a lot on the engine and the mileage — an older engine with 120,000 miles on the original plugs points one direction, a newer engine with fresh plugs points another.

What diagnosis looks like and rough cost

We start by pulling the exact freeze-frame data and any other codes stored alongside P0303, since a cylinder-3-only misfire versus misfires on multiple cylinders tells us different things. From there we typically check spark first, since it's fastest to rule in or out, then look at fuel delivery and compression on that cylinder if spark checks out fine. Diagnosis time varies with how the cause presents itself. A straightforward plug or coil problem might be sorted in under an hour once identified. A compression issue takes longer to confirm and costs more to fix, since it can mean internal engine work.

Because of that spread, we're not going to quote a number here. A plug or coil repair is a modest job. A fuel injector or vacuum leak is more. Anything involving compression or a head gasket is a bigger job and a bigger bill. We'll tell you what we find before we do anything, and if it turns out to be more than a plug swap, we'll walk you through the options rather than just replacing parts and hoping.