A failing ignition coil usually causes a misfire on one cylinder: a rough idle, a stumble when accelerating, a flashing check engine light under load and a cylinder-specific code such as P0301 to P0308, often with a coil circuit code such as P0351. Most modern engines use one coil per cylinder (coil-on-plug), which makes finding a bad coil straightforward.
Bad ignition coil symptoms
- Misfire on one cylinder, felt as shaking or a stumble; see engine misfire.
- Flashing check engine light under hard acceleration; see check engine light.
- Rough idle and stalling; see rough idle.
- Loss of power and hesitation, especially climbing hills or towing.
- Worse fuel economy and a fuel smell from the exhaust.
- Hard starting in damp weather, when a cracked coil boot lets spark leak to ground.
Codes a bad coil sets
| Code | Meaning |
|---|---|
| P0300 | Random or multiple cylinder misfire |
| P0301 to P0308 | Misfire in cylinder 1 to 8 |
| P0351, P0352 | Ignition coil A or B circuit fault |
How to test an ignition coil
- Swap test: move the suspect coil to a neighboring cylinder, clear the codes and drive. If the misfire code moves with the coil (for example from P0303 to P0305), the coil is bad.
- Inspect: look for cracks in the coil body, burnt or carbon-tracked boots and oil in the spark plug tube.
- Measure: some coils can be checked for primary and secondary resistance against specification; many modern coils with built-in drivers can only be checked by swapping or with a scope.
- Check power and the trigger signal at the connector if the coil swaps fine but the cylinder still has no spark.
What causes coils to fail
- Heat and age; coils mounted on top of the engine see constant heat cycles
- Worn spark plugs with large gaps, which force the coil to produce higher voltage
- Oil in the plug tubes from leaking valve cover or tube seals, which softens boots
- Water getting into plug wells after washing the engine or driving through deep water
Replace one coil or all of them?
Replacing only the failed coil is normal practice. If several coils have already failed on a high-mileage engine, some owners replace the full set at once. Replace the spark plug in that cylinder at the same time if it is due; see bad spark plug symptoms. If a cylinder still misfires with a known-good coil and plug, check the injector (P0201) and run a compression test; low compression points to internal engine wear, and our engines page lists replacement engines by code.
More guides: spark plug wires · check engine light flashing · how often to change spark plugs.
Trouble codes: P0353 Ignition Coil C Circuit · P0354 Ignition Coil D Circuit · P0355 Ignition Coil E Circuit · P0356 Ignition Coil F Circuit · P0357 Ignition Coil G Circuit · P0358 Ignition Coil H Circuit · P0350 Ignition Coil Circuit · all OBD2 codes.
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