A technician is diagnosing a vehicle with a complaint of a rough idle and a P0300 DTC (Random/Multiple Cylinder Misfire Detected). The scan tool shows misfires on cylinders 2 and 4. Further investigation reveals that Cylinders 2 and 4 are on the same ignition coil secondary circuit (waste spark system). What is the most likely cause of this specific misfire pattern?
- AA vacuum leak affecting multiple cylinders.
- BA failing ignition coil for cylinders 2 and 4.
- CA faulty crankshaft position (CKP) sensor.
- DA clogged fuel filter.
Show answer & explanationAnswer & explanation
Correct answer: B. A failing ignition coil for cylinders 2 and 4.
In a waste spark ignition system, one ignition coil fires two spark plugs simultaneously: one in a cylinder on its compression stroke (firing stroke) and another in a cylinder on its exhaust stroke (waste spark). If cylinders 2 and 4 are paired to the same coil, a misfire on both indicates a failure of that specific ignition coil or its primary circuit, as it's responsible for providing spark to both cylinders.
Why the other options are wrong
- A. A vacuum leak affecting multiple cylinders might cause misfires, but the specific pairing of cylinders 2 and 4 points more directly to a shared component like an ignition coil, rather than a general air leak.
- C. A faulty CKP sensor would typically cause no-start conditions, erratic RPM, or misfires on all cylinders, as it's critical for timing all ignition events.
- D. A clogged fuel filter would likely affect all cylinders or cause more general performance issues, not specifically misfires on two cylinders paired to one coil.
Waste Spark Misfire Pattern
In a waste spark ignition system, two cylinders are paired to one ignition coil. If both cylinders assigned to a single coil misfire, it strongly indicates a fault with that specific ignition coil or its primary circuit.
- One coil fires two spark plugs.
- Paired cylinders are 360 degrees apart in crank rotation.
- Misfire on both paired cylinders = faulty coil.
Memory trick: Two misfires 'Together' means the 'Coil' is 'Broken' for both.