Ignition System Diagnosis and Repair
Primary and secondary circuits
The ignition system turns battery voltage into a high-voltage spark at the correct cylinder and time. Understand the primary circuit (B+, coil primary winding, module/ignition control, ground), the secondary circuit (coil secondary, plug wire or boot, spark plug), and triggering (CKP/CMP, distributor, or coil-on-plug logic). Systems include distributor, waste-spark, coil-near-plug, and COP. In waste-spark, paired cylinders share a coil and fire simultaneously - one on compression, one on exhaust - so a misfire on one cylinder can be caused by the paired cylinder's plug or wire.
Testing
Use a spark tester that loads the circuit, not a screwdriver. Measure coil primary and secondary resistance, check plug boots for carbon tracks, and inspect plugs for gap, wear, and heat range. Secondary ignition waveforms show burn time and firing voltage; a short burn time or high firing voltage signals a wide gap or lean mixture, while a long burn time suggests a rich mixture.
Traps
Misfire codes (P0300-P0308) identify the symptom, not the cause. Swap-test coils, plugs, and injectors to see whether the misfire follows the part or stays with the cylinder. Do not overlook a mechanical cause - a burned valve or bad lifter produces the same code. Check for an arcing coil boot before condemning a plug.
Worked example
P0302. Swap the cylinder 2 coil with cylinder 1. If the misfire moves to cylinder 1, the coil is faulty; if it stays at cylinder 2, test the plug, injector, compression, and valve train there.