Bad Ignition Coil Symptoms: Signs and How to Confirm
A bad ignition coil usually causes an engine misfire: rough idle, shaking, hesitation when accelerating, and a check engine light with a code such as P0301 to P0308 that names the cylinder. If the check engine light is flashing, the misfire is severe enough to damage the catalytic converter.
Common symptoms
- Check engine light, steady or flashing
- Misfire codes P0300 (random) or P0301–P0308 (specific cylinder)
- Rough or shaky idle
- Hesitation, stumbling or loss of power when accelerating
- Worse fuel economy
- Hard starting, especially in damp weather
- Backfiring or a fuel smell from the exhaust
How to confirm a bad coil (the swap test)
On engines with one coil per cylinder, the simplest diagnostic is the swap test. Note the misfire code, for example P0302 for cylinder 2. With the engine off, swap the coil from cylinder 2 with the coil from cylinder 3, clear the code, and drive.
If the code returns as P0303, the misfire followed the coil and the coil is bad. If it stays on cylinder 2, the problem is more likely the spark plug, a fuel injector or low compression on that cylinder.
What does it cost to fix?Ignition Coil Replacement: $75–$310 See the full estimate →What causes coils to fail
- Worn spark plugs: a wide plug gap forces the coil to produce higher voltage, which overheats it
- Heat and vibration over many years
- Oil in the spark plug wells, usually from a leaking valve cover gasket
- Moisture from engine washing or water leaks
Can you keep driving?
With a steady check engine light and a mild misfire, you can drive carefully to a shop. With a flashing light, reduce speed and load and get it fixed quickly: unburned fuel is entering the exhaust and can overheat the catalytic converter, a much more expensive part.
Frequently asked questions
Should I replace all ignition coils at once?
Usually not. Replace the failed coil, unless the car has very high mileage and the other coils are original.
Should I replace spark plugs with a coil?
If the plugs are near or past their service interval, yes. Worn plugs are a common reason coils fail.
How long do ignition coils last?
Often 100,000 miles or more.
Related repair costs
More guides
- How Long Does a Car Battery Last?
- Bad Alternator Symptoms: 7 Signs Your Alternator Is Failing
- Why Does My Car Shake When Braking?
- How Long Do Brakes Last?
Sources and methodology
Estimates combine typical book labor times, current aftermarket and OEM parts prices, and 2026 US shop labor rates. See exactly how we calculate them.
- U.S. Bureau of Labor Statistics: Automotive Service Technicians and Mechanics (pay and duties)
- Federal Trade Commission: Auto Repair Basics (estimates, warranties, your rights)
- NHTSA: Check for recalls before paying for a repair
- ASE (National Institute for Automotive Service Excellence): technician certification