Ignition System

The ignition system has one job — deliver a spark to the right cylinder at exactly the right instant. That spark has to be hot enough and last long enough to reliably ignite a compressed air-fuel mixture under all operating conditions. Cold starts, wide open throttle, idle, high altitude — the ignition system has to perform every time. Miss once and you feel it as a misfire. Miss repeatedly and you damage the catalytic converter.

Battery provides 12 volts. The ignition coil transforms that 12 volts into 20,000 to 100,000 volts. That high voltage travels to the spark plug. At the spark plug tip, the voltage jumps across a small air gap between the center electrode and the ground electrode. That arc is the spark. The spark ignites the compressed air-fuel mixture and the power stroke begins. Every component in this chain must work correctly or that cylinder does not fire.

Most modern engines use coil-on-plug — COP — an individual ignition coil mounted directly on each spark plug. This eliminates high-voltage spark plug wires and the old distributor. Each coil fires only its own plug on command from the PCM. This gives the PCM individual control of ignition timing and spark energy on each cylinder. If one coil fails, only that cylinder misfires. The others keep firing normally.

The PCM decides exactly when to fire each spark plug based on inputs from the crankshaft position sensor, camshaft position sensor, engine load, RPM, coolant temperature, knock sensor, and other inputs. At low RPM and light load, the spark fires earlier in the compression stroke — called advanced timing — to give the flame front more time to burn the mixture. At high RPM, the spark must fire even earlier because the piston is moving faster. Under heavy load and high cylinder pressure, the PCM retards timing to prevent detonation — also called knock. The knock sensor listens for knock and the PCM pulls timing back if it hears it.

The misfire monitor in the PCM identifies which cylinder is misfiring and stores a specific code — P0301 for cylinder 1, P0302 for cylinder 2, and so on. When you have a cylinder-specific misfire code, swap the coil from that cylinder with the coil from an adjacent cylinder. Clear codes and retest. If the misfire code moves to the new cylinder — the coil is the fault. Replace it. If the misfire code stays on the original cylinder — the coil is not the problem. Now swap the spark plug the same way. If it still stays — check injector, then compression. Always swap before you spend.

Without a crankshaft position sensor signal, the PCM cannot determine where the pistons are in their cycle — no spark, no fuel injection, no start. A failing crankshaft position sensor often fails intermittently — drops the signal when hot, restores when it cools. The vehicle cranks normally but will not fire, then starts normally an hour later after sitting. Always test the crank sensor signal waveform during a confirmed no-start event before condemning any other component.