Cylinder 4 Misfire Detected
Cylinder 4 isn't firing properly—think of it like one cylinder in a 4-cylinder engine refusing to do its job. The engine detects inconsistent combustion in that specific cylinder, causing rough running and power loss.
The ECU monitors crankshaft speed variation between cylinder firings using the crankshaft position sensor. When cylinder 4 misfires, it causes irregular acceleration and deceleration of the crankshaft. The ECM detects this deviation from expected combustion patterns and triggers the fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Crankshaft acceleration variance | Consistent 1-2% variance between cylinders | Exceeds 5-10% variance on cylinder 4 |
| Ignition timing/combustion stability | Stable fuel burn within ±5° crank angle | Unstable or missing combustion events |
Code P0305 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0305 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.