P0304

What P0304 means

The computer tracks how smoothly each cylinder spins the crankshaft. Cylinder 4 is losing power on its combustion stroke, either not firing at all or firing weak. The code doesn't say why. It just says that cylinder isn't pulling its weight.

Is it safe to drive

Not for long. A misfire dumps raw, 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 in the first place. If the check engine light is flashing, that's the car telling you the misfire is active right now. Stop driving it and get it looked at. If the light is steady, you have more time, but don't put it off for weeks. Watch for a rough idle, a shudder under load, hesitation when you accelerate, or a smell of raw fuel from the tailpipe.

Common causes, most likely first

On most gas engines, the spark plug or ignition coil for cylinder 4 is the first suspect, especially if the car is due for a plug change or has over 60,000 miles on the plugs. Many modern engines use one coil per cylinder, so a coil that's failing on cylinder 4 only will throw exactly this code.

After that, look at a vacuum leak or a clogged fuel injector on that cylinder, which starves it of the right air-fuel mix. Low compression from a worn valve or a leaking head gasket will also cause a persistent misfire, but that's less common and shows up more on higher-mileage engines. On direct-injection engines, carbon buildup on the intake valves can also be a factor. Which of these is likeliest depends a lot on the specific engine and how many miles are on it, so we don't guess before we've pulled data.

What diagnosis involves and what it tends to cost

We start by pulling the freeze frame data and misfire counts from the computer, then usually swap the coil from cylinder 4 to another cylinder to see if the misfire follows the part or stays put. That tells us right away whether it's the coil or something downstream. From there we'll check the spark plug, look for a vacuum leak, and if needed run a compression or fuel injector test on that cylinder.

A coil and plug replacement on one cylinder is a modest repair, often in the range of a couple hundred dollars including parts and labor, though it varies by engine and how hard the plug is to reach. If it turns out to be a fuel injector or a compression problem, the cost goes up from there, sometimes well above that, depending on what's found. We won't know which one you're dealing with until we've actually tested it, and we'd rather tell you that than quote a number that doesn't hold up once the car's on the lift.

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.