If your engine misfires, idles roughly, loses power on hills, and the check engine light has come on, a failing ignition coil is one of the most likely causes, and the usual fix is to replace the ignition coil on the cylinder that is misfiring. Ignition coils wear out with heat and age, and most drivers meet this repair somewhere past 100,000 miles. The good news is that diagnosis is straightforward and the job itself is one of the simpler engine repairs.
What an ignition coil does
Your battery supplies only 12 volts, but a spark plug needs a burst of 20,000 volts or more to fire. The ignition coil is a small transformer that creates that burst. Most modern engines use one coil per cylinder, mounted directly on top of each spark plug in a design called coil on plug. Older engines may use a single coil pack shared by several cylinders. Every time the engine computer signals, the coil fires its plug, and the fuel and air mixture burns. When a coil weakens, its cylinder stops contributing, and the engine runs on the remaining cylinders.
Signs it is time to replace the ignition coil
- Engine misfires: the car jerks, stumbles, or hesitates, especially when accelerating or climbing a hill
- Rough idle: the engine shakes and sounds uneven when stopped at a light
- Check engine light: misfire codes are the single most common trigger for the light on petrol engines
- Loss of power: the car feels flat and unwilling, as if it is towing something heavy
- Worse fuel economy: unburned fuel passes through the dead cylinder, so the engine drinks more
- Hard starting: weak or missing spark makes cold starts slow and cranky
- Backfiring or a fuel smell: unburned fuel can ignite in the exhaust, which can damage the catalytic converter
These complaints overlap with worn spark plugs, which produce almost identical misfires. Before buying coils, read our guide to the signs of bad spark plugs, because tired plugs are cheaper to replace and often the true cause.
How to confirm the coil is really the problem
A code reader is the fastest route. Misfire codes P0300 through P0304 and beyond name the cylinder that is missing, and codes such as P0351 point directly at a coil circuit fault. On a coil on plug engine there is a simple home test: swap the suspect coil with the coil from a healthy cylinder, clear the codes, and drive. If the misfire moves to the other cylinder, the coil is guilty. If the misfire stays put, the plug, injector, or compression on that cylinder needs attention instead.
A visual check helps too. Cracked coil bodies, white tracking marks, oil sitting in the plug tube, and corroded connectors all condemn a coil without any further testing.
What the replacement job involves
On most four and six cylinder engines the coils sit on top of the valve cover under a plastic engine cover, and the job goes like this.
- Let the engine cool fully and disconnect the negative battery cable
- Remove the engine cover and unplug the coil connector
- Unbolt the coil hold down bolt and pull the coil straight up off the plug
- Push the new coil firmly into place, refit the bolt, and reconnect the plug
- Reconnect the battery, start the engine, and clear the stored codes
Safety matters here. Ignition systems produce very high voltage, so never pull a coil or plug wire off a running engine, and always work on a cool engine with the battery disconnected. On some V6 and V8 engines the rear coils hide under the intake manifold, which turns a quick job into a long one and is the main reason labor prices vary so widely.
Ignition coil replacement cost
| Cost item | Typical range |
|---|---|
| Single coil on plug part | $35 to $120 |
| Full set of coils | $150 to $450 |
| Older style coil pack part | $80 to $250 |
| Labor for an accessible engine | $50 to $150 |
| Total for one coil, fitted | $120 to $350 |
Prices vary by car model and region. Because the labor to reach the coils is often the largest part of the bill, many owners replace the spark plugs during the same visit while everything is already apart. If your car is due for plugs anyway, our iridium spark plugs guide explains why long life iridium plugs are usually worth their small price premium.
Can you replace an ignition coil yourself
On engines where the coils sit in the open on top of the valve cover, this is one of the most beginner friendly repairs in the engine bay, often needing nothing more than a socket set and 30 minutes. Doing it yourself saves the labor charge entirely, which is why many owners tackle a single failed coil at home. The jobs to leave to a workshop are the ones where the intake manifold must come off for access, because a slipped gasket or broken bolt turns a cheap repair into an expensive one. While each coil is out, inspect its rubber boot for cracks and tears, since a damaged boot can leak spark and mimic a failed coil.
Frequently asked questions
Should I replace all the ignition coils at once?
If one coil has failed from simple age on a high mileage engine, the others are usually close behind, so replacing the full set prevents repeat labor charges. If a single coil failed early from damage or oil contamination, replacing just that one and fixing the leak is reasonable.
How long do ignition coils last?
Most coils last 100,000 to 150,000 miles. Heat, worn spark plugs with wide gaps, and oil leaking into the plug tubes all shorten coil life, because the coil must work harder to fire the plug.
Can I drive with a bad ignition coil?
Short, gentle trips are possible, but it is a poor idea for long. A dead cylinder washes oil from its walls and sends unburned fuel into the exhaust, which can overheat and destroy the catalytic converter, a far more expensive part than any coil.
A misfiring engine is telling you exactly where it hurts, and a code reader plus the simple swap test will confirm a bad coil in minutes. Fix it promptly and the car usually returns to smooth, efficient running the same day.