P0303

What P0303 means

The computer tracks how smoothly the engine spins between firing pulses. When cylinder 3 fires late, weak, or not at all, the crankshaft hiccups for an instant and the computer logs it. That's the whole code. It does not know why cylinder 3 is misfiring, only that it is.

Is it safe to drive

Say so plainly: a steady misfire on one cylinder can be driven short distances, but it's not something to put off. Unburned fuel from a misfiring cylinder washes down the cylinder wall and can get into the oil, and it also raises the exhaust temperature at the catalytic converter, which is expensive to replace. If the check engine light is flashing, not just on steady, that's the car telling you to stop soon. A flashing light means the misfire is severe enough to damage the converter quickly. Watch for a rough idle, a shudder under load, or a smell of raw fuel from the exhaust. Any of those, get it looked at within a few days rather than a few weeks.

Common causes, most likely first

On most engines, a worn or fouled spark plug is the most common cause. Next most common is an ignition coil going bad, either the coil for that cylinder on a coil-on-plug setup or a coil pack shared between cylinders. After that: a fuel injector that's clogged or not opening right, a vacuum leak near that cylinder's intake runner, or low compression from a valve or piston ring problem. On direct-injection engines carbon buildup on the intake valves can also cause a lean misfire that looks the same on a scanner. It's worth saying that a misfire code often points at more than one cylinder before it settles on cylinder 3 specifically, and if other cylinders are also listed, that usually means something feeding the whole engine, like fuel pressure or a vacuum leak, rather than one bad part.

What diagnosis involves and what it tends to cost

A misfire code is a starting point, not an answer, so diagnosis usually means pulling the plug and coil from cylinder 3 to check their condition, testing the injector, and checking compression to rule out a mechanical problem before replacing anything. Swapping the coil or plug from a known-good cylinder into cylinder 3 is a quick way to see if the misfire moves with the part.

Cost depends entirely on what's actually wrong. A plug or coil is usually a small job, often under $200 depending on the engine and how many cylinders end up needing plugs at once. An injector or a compression problem costs more, and how much more depends on what the engine needs. We won't quote a number without looking at it first. If you're in Puyallup, Spanaway, or anywhere else nearby and want it checked, we're open Monday through Friday, 8 to 5, and can get it on the scanner before anything gets replaced.

Generic OBD-II guidance — manufacturer-specific codes and causes vary by make, so always confirm against your vehicle. A code identifies a system or condition; it does not, on its own, prove which part has failed.