Ignition Coil

The ignition coil is the high-voltage transformer in the petrol engine’s ignition system that steps up the battery’s 12-volt supply to the 15,000–45,000 volts required to fire a spark across the spark plug gap under compression. On modern engines, coil-on-plug (COP) designs place a dedicated ignition coil directly over each spark plug, eliminating the need for a distributor and plug wires and providing precise, individually-controlled spark timing for each cylinder. Earlier designs used a single coil with a mechanical distributor that rotated to deliver the high-voltage pulse to each plug in the firing order, or a coil pack serving pairs of cylinders. The coil’s primary winding — a few hundred turns of relatively thick wire — carries the 12-volt switching current controlled by the ECU through the ignition transistor. When this primary current is interrupted at the correct ignition timing moment, the collapsing magnetic field in the coil’s iron core induces a very high voltage in the secondary winding (several thousand turns of fine wire) by transformer action. The ratio of secondary to primary turns determines the output voltage multiplication. Ignition coil failures are among the most common causes of check engine lights and misfires on high-mileage engines, with coil-on-plug designs being particularly susceptible to the heat and oil environment at the top of the engine.

The ECU precisely times the interruption of each coil’s primary current to fire the spark at the optimal advance angle before top dead centre for the current engine speed, load, and temperature conditions. On COP systems, each coil’s firing is controlled entirely independently, allowing the ECU to apply cylinder-individual timing corrections based on knock sensor feedback, misfire detection, and individual cylinder charge density variations. The coil must charge and fire repeatedly at rates up to once per crankshaft revolution at high RPM — on a four-cylinder engine running at 6,000 RPM, each coil fires 50 times per second. The dwell angle — the period during which the primary current charges the coil before firing — determines the energy stored in the coil’s magnetic field and therefore the spark energy available. The ECU continuously adjusts dwell to ensure maximum coil energy across all operating conditions, compensating for variations in battery voltage. A COP coil failure affects only one cylinder; older distributor-based single-coil failures affect all cylinders simultaneously and are immediately apparent as a complete no-start or severe rough running.

Coil failure causing single-cylinder misfire: COP coil failure is the most common ignition fault on modern engines. The coil’s internal primary or secondary winding develops an open circuit or internal short, causing that cylinder to misfire permanently. The check engine light illuminates with a misfire code and a coil circuit code for the specific cylinder. Symptoms are rough idle, one-cylinder power loss, and increased fuel consumption. Confirming the fault by swapping the suspect coil to a different cylinder and checking if the misfire code follows the coil confirms the diagnosis without purchasing parts unnecessarily.

Oil contamination of COP coil from valve cover seal failure: COP coils sit in the spark plug wells that pass through the valve cover. A failed spark plug well seal allows engine oil to pool in the well and eventually immerse the coil boot and coil base. Oil degrades the coil’s insulation, causing internal arcing and coil failure. Any multiple-coil failures on the same engine, or a pattern of repeated coil failures, should prompt inspection of the plug well seals as the root cause.

High-resistance coil boot causing misfire under load: The rubber boot that seals the COP coil’s high-voltage output to the spark plug can develop cracks that allow the high-voltage spark to arc to the cylinder head rather than firing across the plug gap, particularly under the high pressures of wide-open-throttle operation. The engine runs normally at light load but misfires under full acceleration, which is a distinctive symptom of a leaking coil boot or cracked coil tower.

Level 1 — Check Engine Light and Misfire Diagnosis: A flashing check engine light indicates an active misfire that is damaging the catalytic converter — reduce engine load immediately and have the fault code read within the same journey. A steady check engine light with a rough idle suggests an intermittent or permanent misfire. The fault code will identify the specific cylinder; the coil on that cylinder is the primary suspect. Before replacing the coil, confirm by swapping it with an adjacent cylinder’s coil and rechecking whether the misfire code follows the coil or stays at the original cylinder number.

Level 2 — Coil Replacement at Spark Plug Service Interval: Consider replacing all COP coils simultaneously when replacing spark plugs on a high-mileage engine — the coils have experienced the same thermal and electrical stress for the same mileage and tend to fail in sequence as they age. If one coil has already failed, the others are likely near their end of service life. This combined replacement prevents multiple individual return visits to the workshop within the next service interval. Always inspect the plug well seals when accessing the coils — replace any that show oil contamination.

Level 3 — Ignition System Testing Under Load: A coil that appears functional at idle but fails under full-throttle load is difficult to diagnose without a running under-load test or an oscilloscope connected to the primary circuit. An automotive oscilloscope showing the coil primary waveform reveals weak spark, short dwell, or intermittent primary opens that a simple resistance measurement misses. Professional ignition system diagnosis is worthwhile before replacing multiple coils based on intermittent symptoms.