How to test · Actuator

How to test an ignition coil: supply, trigger signal, resistance and secondary waveform

A failing coil causes a misfire on one cylinder, often only under load or in damp weather, and a flashing check engine light. Coils fail from heat, age and from working against worn spark plugs with wide gaps, which need a higher voltage. Always check the spark plug and the boot when you find a bad coil.

Updated 25 Oct 2026 · written by ipacen; typical values, not manufacturer data

In short

Most modern cars use coil-on-plug units with a built-in driver: check for a 12 V supply, a good ground and a trigger signal from the ECU that switches between about 0 V and 5 V. Coils without a driver can be measured: primary typically about 0.3–1 Ω, secondary about 6–15 kΩ. The surest test is the scope: a firing line of roughly 5–15 kV at idle and a spark line of about 1–2 ms. A quick workshop check is to swap the coil to another cylinder and see if the misfire follows.

Typical values

Check	Typical good	Suspect
Primary resistance (coils without driver)	About 0.3–1 Ω	Open or shorted
Secondary resistance (coils with towers)	About 6–15 kΩ	Open or very high
Firing line (secondary, idle)	About 5–15 kV, even between cylinders	Much higher (worn plug, open) or much lower (fouled plug, tracking)
Spark burn time	About 1–2 ms	Short (high resistance) or very long
Trigger signal (coil-on-plug)	Clean 0–5 V pulses for every firing	Missing: ECU driver or wiring

Typical ranges, not manufacturer data: check the specification for the engine.

Which type is it?

Coil-on-plug with built-in driverThree or four wires: supply, ground, trigger from the ECU (and sometimes a feedback line). Resistance cannot be measured meaningfully; test supply, ground, trigger and the spark.

Coil without driver (coil packs, older coils)Two primary wires: supply and the switched side. Primary and secondary resistance can be measured.

Wiring: what you should measure

Coil-on-plug with driver

Wire	What you should see
Supply	12 V with the ignition on
Ground	Less than about 0.1 V drop under load
Trigger from the ECU	0–5 V square pulses, dwell typically about 2–5 ms

Coil without driver

Wire	What you should see
Primary +	12 V with the ignition on
Primary −	Switched to ground by the ECU or igniter; a spike of roughly 300–400 V when it opens

Before you start

Read misfire counters per cylinder: one cylinder points to coil, plug, injector or compression on that cylinder.
Remove the coil and check the boot and plug well: carbon tracks, cracks, water or oil.
Check the spark plug condition and gap.

Swap test

Swap the suspect coil with the coil from another cylinder and clear the counters.
If the misfire moves with the coil, the coil is faulty; if it stays, look at the plug, injector and compression.

Multimeter

Measure the supply and ground at the coil connector.
On coils without a driver, measure primary and secondary resistance and compare with the specification.

Oscilloscope

Secondary: a capacitive probe on top of the coil, about 5 kV/div and 0.5 ms/div at idle; compare firing lines and burn times across cylinders.
Trigger: back-probe the ECU trigger wire, 0–10 V, and check a clean pulse for every firing.

What it looks like on a scope

Good5 ms (idle)

Secondary at idle: a firing line of about 10 kV, a spark line of about 1.5 kV lasting about 1.5 ms, then the coil oscillations.

Fault: High firing line, short burn5 ms (idle)

Firing line near 19 kV and a short spark: a worn plug with a wide gap or high resistance in the secondary. The coil is working hard and will fail early.

Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.

What the readings tell you

You find	It usually means
Misfire follows the coil after a swap	Faulty coil
Misfire stays with the cylinder	Spark plug, injector or compression on that cylinder
High firing line, short burn time	Worn plug with a wide gap, or high resistance in the secondary
Low firing line, long burn	Fouled plug, or low compression on that cylinder
No trigger pulses	ECU driver or wiring fault

Known engine problems behind these codes

1.2/1.4 TSI EA111: Cracked pistons on the high-output 1.4 TSIVolkswagen, Audi, Skoda, Seat · typically 80,000–180,000 km 1.8/2.0 TFSI EA888: High oil consumption (piston rings)Audi, Volkswagen, Skoda, Seat · typically 60,000–160,000 km 1.8/2.0 TFSI EA888: Carbon build-up on the intake valvesAudi, Volkswagen, Skoda, Seat · typically 80,000–160,000 km 1.2 PureTech: High oil consumptionPeugeot, Citroën, DS, Opel · typically 60,000–160,000 km 1.6 THP/Prince: Oil consumption and carbon build-upMini, BMW, Peugeot, Citroën, DS · typically 60,000–160,000 km 1.2 TCe: High oil consumptionRenault, Dacia, Nissan, Mercedes-Benz · typically 40,000–120,000 km Theta II 2.0/2.4: Carbon build-up on the intake valves (GDI)Hyundai, Kia · typically 80,000–180,000 km MultiAir/TwinAir: MultiAir actuator failureFiat, Alfa Romeo, Lancia · typically 80,000–180,000 km

Questions mechanics ask

How do I test a coil-on-plug ignition coil?

Check the 12 V supply, the ground and the trigger signal from the ECU, then check the spark with a spark tester or a secondary scope probe. The fastest workshop test is swapping the coil with another cylinder and seeing if the misfire follows.

What resistance should an ignition coil have?

Coils without a built-in driver typically measure about 0.3–1 Ω on the primary and about 6–15 kΩ on the secondary. Coils with a built-in driver cannot be checked by resistance; use the supply, trigger and spark tests.

Can worn spark plugs damage ignition coils?

Yes. A wider gap needs a higher voltage to fire, which stresses the coil insulation and the boot. Replace worn plugs when you replace a coil.

Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.