Coil packs (ignition coils) step 12V up to 30,000+ volts to fire the spark plugs. When one fails, you get a hard misfire on one or more cylinders. Here are the 7 most common signs of a bad coil pack and what replacement costs.
A failed coil produces a hard, steady misfire on one cylinder - usually P0301-P0308. The engine shakes at idle and feels weak.
A flashing CEL means active misfire that is damaging the catalytic converter. Coil failure is the most common cause.
With one cylinder dead, the engine feels like it is running on three (or seven, on a V8). The shake is most obvious at idle and at low RPM.
Under load, a weak coil drops spark to the failing cylinder. You feel a stumble or buck when you press the throttle.
A coil weak enough to misfire under load may still fire when cold but struggles to keep up during cranking, especially in cold weather.
Misfires dump unburned fuel into the exhaust. MPG drops noticeably and the exhaust may smell strongly of gas.
Long-term untreated misfire torches the cat. You will eventually see a P0420 code and need a much more expensive repair.
Symptoms overlap between parts. Run through these top 3 confirming tests before spending money on parts:
Costs vary by vehicle make, model year, and parts quality. Always get a written estimate before authorizing work.
On most modern engines a coil-on-plug is one bolt and one electrical connector per coil. The hardest part is sometimes accessing the rear bank on a transverse V6.
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If your scan tool shows one of these codes, you can confirm the diagnosis. Click for full code details, common causes, and repair guidance.
If you have over 80,000 miles, replacing all of them at once is a good preventive move. Otherwise, replacing just the failed coil plus the matching spark plug is fine.
Typically 80,000 to 150,000 miles. Heat, oil leaks at the valve cover, and bad spark plugs (high resistance) all shorten coil life.
Yes - prolonged misfires destroy catalytic converters and can wash oil off cylinder walls from raw fuel. Fix it quickly.
Strongly recommended. A worn plug is often what killed the coil, and a new coil with old plugs may fail again in months.
Yes - flashing CEL = active misfire = real catalytic converter damage. Drive gently to a shop or driveway and replace it before it kills the cat.
Quality OEM-equivalent brands (Denso, NGK, Bosch, Delphi) are reliable. Avoid no-name parts on coil packs - cheap coils tend to fail within 6 months.