P0307 - Cylinder 7 Misfire Detected
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0307 is an OBD-II diagnostic trouble code indicating that the Engine Control Unit (ECU) has detected a misfire on the seventh cylinder. The ECU monitors the crankshaft speed variations using the Crankshaft Position (CKP) sensor. A misfire results in a momentary decrease in crankshaft rotational acceleration during that cylinder's power stroke. If the deviation exceeds the calibrated threshold for emissions or catalyst protection, the code is stored. A flashing MIL indicates a catalyst-damaging misfire. The ECU may disable the fuel injector for cylinder 7 as a fail-safe measure.
Possible Causes
Faulty ignition coil (COP) or spark plug on cylinder 7
Failing fuel injector or clogged injector nozzle
Intake manifold vacuum leak at cylinder 7 or low engine compression (mechanical issue)
Damaged spark plug wire (on older systems) or internal ECU driver circuit failure
Symptoms
Recommended Solution
- VISUAL INSPECTION: Check ignition coil and connector for cylinder 7 for corrosion, oil in the spark plug well, or damaged wiring.
- SWAP TEST: Switch the ignition coil from cylinder 7 to a known good cylinder (e.g., cylinder 1). Clear codes and retest. If the code moves to P0301, the coil is faulty.
- MEASUREMENT: Inspect the spark plug (Gap: usually 0.032"-0.044", Resistance: 3k-8k Ohms). Verify 12V power supply to the coil and check ground circuits.
- ADVANCED DIAGNOSIS: Perform a compression test or leak-down test. Use an oscilloscope to monitor injector pulse width and ignition primary/secondary patterns.
- ACTION: Replace faulty spark plug, coil, or injector. Address mechanical issues if compression is low.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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