Each ignition coil sends 30,000+ volts to its spark plug to fire the cylinder. When a coil fails, that cylinder doesn't fire at all - so you get a hard misfire, rough running, and a P0301+ code pointing right at the dead cylinder. Here are the 7 warning signs.
A code like P0303 (cylinder 3 misfire) is the dead-giveaway for a coil. Swap the suspect coil with a known-good one and watch if the code follows the coil.
These codes specifically call out the coil's electrical circuit. P0351 = coil 1 fault. Less common than P0301-style codes but a direct indicator.
A dead coil = a dead cylinder. The engine shakes hard, exhausts a strong gas smell, and feels like it's running on 3 instead of 4 (or 5/6, etc).
On engines with a single coil pack (older cars), a failing coil prevents start entirely. On coil-on-plug systems, you'll start but run very rough.
Unburned fuel from the dead cylinder dumps into the exhaust. You'll smell raw gasoline standing behind the car.
An active misfire flashes the CEL. Each unfired cycle dumps raw fuel into your catalytic converter, which can melt it ($1,500+ damage). Pull over and tow.
A dead cylinder takes 25% of your power on a 4-cylinder, 16% on a V6. Acceleration is sluggish and you may have trouble climbing hills.
Symptoms overlap between parts. Run through these checks before spending money on parts:
On modern coil-on-plug cars, you can usually replace one coil. Some shops recommend doing all of them when one fails - reasonable on high-mileage cars but not strictly necessary.
Coil-on-plug replacement is one of the easiest DIY jobs: pull the connector, remove one bolt, lift out the old coil, drop in the new one. 5-10 minutes per coil with hand tools.
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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.
Modern coil-on-plug units typically last 80,000 to 120,000 miles. Heat, vibration, and worn spark plugs (which force the coil to work harder) shorten life.
Briefly, only if the CEL is solid. A flashing CEL = active cat damage - get towed or limp home and don't drive again until repaired.
Not required. If your car has 100k+ miles and the coils are original, replacing all of them at once saves future labor. Otherwise, just replace the bad one.
Yes, if the plugs are due or you don't know the last replacement. Worn plugs are often what kills coils in the first place. They're cheap insurance.