Ignition Coil Failure: Diagnosis, Symptoms & Replacement Guide

Modern vehicles use one ignition coil per cylinder (Coil-On-Plug/COP). Coil failure is among the top 10 most common repair reasons, making ignition coils a high-demand, high-margin product line.

How COP Coils Work

Each COP unit sits directly on the spark plug, stepping up 12V to 25,000-45,000 volts. The ECM triggers each coil individually at the precise moment for its cylinder. Heat, vibration, and voltage stress cause insulation breakdown over time.

Failure Symptoms

Engine misfire is the primary symptom — rough idle, hesitation, reduced power. On a 4-cylinder engine, one failed coil causes 25% power loss. Check engine light with codes P0300 (random misfire) or P0301-P0312 (cylinder-specific) pinpoint the coil. Fuel consumption increases 10-20%.

Diagnostic Testing

OBD-II scan identifies the cylinder. Swap the suspected coil with an adjacent cylinder — if the misfire follows, the coil is confirmed bad. Resistance test: primary winding 0.5-2.0 ohms, secondary 6,000-15,000 ohms.

Market Opportunity

Typical life 80,000-120,000 km. Many mechanics replace all coils on the same bank — a 4-coil sale from one diagnosis. OE coils $40-80 each, aftermarket $10-25. High-demand applications: Toyota 90919-02258, VW/Audi 06H 905 110, Honda 30520-5A2-A01, BMW 12 13 7 575 010.

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