P0309
Cylinder 8 Misfire Detected
Powertrain Ignition System Cylinder Misfire 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

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.

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Symptoms You May Notice
3 known symptoms for this code
Rough idle or engine vibration
Loss of power and poor acceleration
Check Engine Light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

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.

Voltage & Parameter Thresholds

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
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Spark Plug
Replace the spark plug in cylinder 8; worn or fouled plugs are the most common cause.
2
Ignition Coil
Swap or test the ignition coil on cylinder 8 to rule out coil failure.
3
Fuel Injector
Have cylinder 8 fuel injector tested or cleaned if fuel delivery is suspected as the culprit.