Cylinder 8 Misfire Detected
Cylinder 8 isn't firing properly, like one spark plug in an 8-cylinder engine misfiring repeatedly. This causes the engine to run rough and lose power because that cylinder isn't contributing to combustion.
The ECU monitors crankshaft speed variation using the crank sensor to detect if cylinder 8 is producing consistent power strokes. When a misfire occurs, that cylinder doesn't fire, causing a brief dip in engine speed that the ECU detects. If misfire count exceeds the threshold over a set number of cycles, the fault is logged.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Cylinder 8 Misfire Count | 0-2 misfires per 1000 combustion cycles | >2-3 misfires per 1000 cycles (varies by OEM) |
| Crankshaft Speed Variance | ±50 RPM between cylinders | >±100 RPM deviation on cylinder 8 |
Code P0309 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, P0309 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.