What P0304 means
| What the code meansSAE J2012 definition | Cylinder 4 Misfire Detected |
|---|---|
| Code | P0304 |
| Areathe engine, transmission and the emissions equipment attached to them | Powertrain |
| System | Ignition system or misfire |
| Defined bythe second character is "0", which marks a generic code | SAE — the same on every make |
| Warning lighta flashing light on a misfire code means stop driving | May flash |
Notes
- If the check engine light is flashing rather than steady, stop driving. A flashing light on a misfire code means unburnt fuel is reaching the catalytic converter, which can destroy it in minutes — a repair an order of magnitude more expensive than whatever caused the misfire. Pull over safely and have the vehicle recovered. A steady light with the same code is not that urgent.
- This is a generic code, so the definition above is the same on every make. What differs between manufacturers is not what the code means but what commonly causes it and how the fault is confirmed — which is why the marque-specific service information still matters even for a code any scanner can read.
- Read the freeze frame before clearing anything. The module stores the conditions at the moment the code set — engine speed, load, coolant temperature, fuel trims, vehicle speed — and that snapshot is usually more diagnostic than the code itself. Clearing the code erases it, along with the readiness monitors, and a vehicle with incomplete monitors will fail an emissions test even with no light on.
- One code often causes another. A single upstream fault — an intake leak, a weak fuel pump, low coolant — can set several codes across different systems. Where more than one is stored, the oldest and the most upstream is the one to work from, not the one with the most alarming description.
What P0304 means
P0304 — Cylinder 4 Misfire Detected.
It is a generic Powertrain code, reported by Ignition system or misfire.
Reading a code without chasing the wrong part
A diagnostic trouble code names the circuit that reported a problem, not the component that failed. The distinction is where most money gets wasted: a code naming a sensor is set by the module that reads that sensor, and the module cannot tell the difference between a failed sensor, a broken wire, a corroded connector, and a sensor faithfully reporting a condition that really is out of range.
The order that works is the same every time. Read the freeze frame first — the snapshot of engine speed, load, coolant temperature and fuel trims at the moment the code set. Check for the simple physical causes that produce this family of codes. Then test the circuit before replacing anything in it.
Generic and manufacturer codes are not the same kind of thing
The second character of the code says which it is. 0 and 2 mean the definition comes from SAE and is identical on every vehicle sold in the US since 1996. 1 and 3 mean the manufacturer defined it, and the same five characters can mean completely different things on two different marques.
That is why a scanner can read any car's generic codes but the marque's own service information still matters — and why a definition copied from a forum for a different make is a genuinely dangerous starting point.