P0304
Cylinder 4 Misfire Detected
Encountering the engine check light code P0304 signifies an explicit mechanical or electrical operational breakdown categorized as "Cylinder 4 Misfire Detected". Professional scanner tools usually flag this subsystem loop early on.
Driver's Summary
A P0304 fault code points directly to a problem with cylinder 4 misfire detected that the ECM has confirmed over multiple drive cycles. Drivers typically experience stalling at stops, poor fuel economy, rough running when this code is active. This condition is classified as high severity. Prompt diagnosis is essential to prevent cascading damage to related components.
Symptoms
Stalling at stops, poor fuel economy, rough running
Common Causes
- Worn spark plug
- Faulty coil
- Injection circuit issue
- Head gasket leak
How to Fix
- 1 Replace spark plugs
- 2 Replace ignition coil
- 3 Test injector wiring
- 4 Check coolant for oil contamination
Technical Explanation
Detection of P0304 occurs when the ECM cross-references multiple sensor inputs and determines that the reported values are physically inconsistent or out-of-range. Misfire rate is counted per cylinder over rolling windows and compared against two thresholds: a catalyst-damaging rate (triggers flashing MIL) and an emissions-exceeding rate (triggers solid MIL). The PCM logs which cylinder is misfiring based on crankshaft position at the time of each detected event. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
An active P0304 code under high-severity conditions means the affected system is operating outside safe parameters. Continued driving — especially under load or at highway speeds — significantly increases the risk of secondary damage to components like faulty coil.
Mechanic's Pro Tip
The fastest isolation technique for P0304 is the coil swap test: move the ignition coil from the affected cylinder to a neighboring cylinder and clear the code. If the misfire follows the coil, it's the coil. If it stays on the same cylinder, focus on the spark plug, injector, or compression. Never replace coils without also replacing the spark plug in that cylinder — a fouled plug will kill a new coil within weeks.
Coil/Plug: $150; Head gasket: $1,200+