Ignition Coil Replacement: Misfires, Testing and Cost

What does an ignition coil do?

An ignition coil turns the car's low battery voltage into the very high voltage a spark plug needs to fire. Standard Motor Products, which makes ignition parts, describes coil-on-plug assemblies as converting a low voltage on the primary side to a high voltage on the secondary side to fire the plugs.

Designs vary. NGK lists everything from older distributor-based systems to distributorless coils, coil-on-plug or pencil coils that each provide energy for a single spark plug, and rail coils that combine several coils in one assembly. On a coil-on-plug engine, one failed coil usually affects one cylinder. On a rail coil, the whole assembly may be replaced as one part.

What are the signs of a bad ignition coil?

Standard Motor Products lists engine misfires, rough idle, a loss of power when accelerating, poor fuel economy and a check engine light as signs of a failed ignition coil. NGK names misfire codes and poor fuel efficiency as the usual clues.

In everyday terms, drivers tend to notice:

• A shake or stumble at idle, often worse at a stop light.
• Hesitation or jerking when you press the gas, especially going uphill or merging.
• A steady or flashing check engine light.
• Worse fuel economy than usual.
• A failed emissions test. AAA explains that an ignition problem such as a misfire can send unburnt fuel through the exhaust, which can lead to an emissions test failure.

If the car is shaking while stopped, see our guide to a car shaking at idle.

Our guide to a flashing check engine light covers what to do in the moment.

Is it the coil, the plug or something else?

A misfire has several possible causes, and the coil is only one of them. Worn or fouled spark plugs, plug wires on older engines, fuel injectors, air leaks and, less often, mechanical engine problems can all cause the same shake. That is why a good mechanic confirms the cause before replacing parts.

Coils and plugs are also linked. AAA notes that a technician servicing spark plugs can spot underlying issues such as oil leaks, misfires or ignition coil problems that contribute to plug wear. NGK lists flash-over, corrosion on the coil terminals and temperature extremes, including moisture that freezes and cracks the coil, among the causes of coil failure. Standard Motor Products adds that missing or poor chassis and body grounds can cause coils to fail the first time and again after replacement.

Can a mobile mechanic replace ignition coils in the driveway?

Usually, yes. Coil replacement is common mobile work and rarely needs a lift. On many engines, coil-on-plug units sit on top of the engine under a plastic cover: the mechanic unplugs the connector, removes a bolt or two and lifts the coil out.

Access is the main variable. On some engines, one bank of coils sits under other parts, such as the intake, which adds time and labor. Give the mechanic your exact year, make, model and engine so they can check before they arrive.

Where a driveway visit reaches its limit is the root cause. If testing points to low compression or another internal engine problem rather than the coil, that is shop work. See what a mobile mechanic can and can't do.

What does the job involve?

1. Read the codes. The mechanic connects a scan tool to the OBD-II port. AAA describes these scanners as tools that help mechanics find problems within the engine, exhaust system and other systems. The codes and freeze frame data show which cylinder is misfiring and under what conditions.
2. Inspect and test. They look for oil or water in the plug wells, damaged boots and corroded connectors. A common check is to swap the suspect coil to another cylinder and see whether the misfire follows it.
3. Quote. You get a price for the coil or coils, and for plugs if they are due, before work starts.
4. Replace. The old coil comes out and the new one goes in. If plugs are being replaced, they are fitted at the same time.
5. Clear and verify. The mechanic clears the codes, runs the engine and road-tests the car to confirm the misfire is gone.

One replacement choice to discuss: Standard Motor Products recommends replacing the spark plugs and the other ignition coils when a coil has failed, to prevent repeat visits. Replacing only the failed coil costs less today. Ask the mechanic what they recommend for your engine and why, and when your plugs are due under your owner's manual. See spark plug replacement.

How much does ignition coil replacement cost?

This site's standard price table does not include a separate row for ignition coils, so the mechanic quotes after looking. The price depends on how many coils you need, how hard they are to reach, and whether the parts are OEM or aftermarket. These related charges come from the table, based on prices that mechanics we list publish on their own websites:

Prices vary by vehicle and city. Ask for a written estimate that lists the coils and plugs separately. See mobile mechanic cost for the full table.

What should you ask before booking?

• Which cylinder is misfiring, and how did you confirm it is the coil rather than the plug or injector?
• Is the quote for one coil or a full set? Are plugs included?
• Are the coils OEM or aftermarket, and what warranty covers them?
• Will you check the grounds and connectors so the new coil doesn't fail the same way?
• Is there a trip fee, and is the diagnostic credited if you do the repair today?

A mechanic who can see the car will know more than any guide. For more on choosing someone, see hiring a mobile mechanic.

What to do next

If the light is flashing or the car is shaking badly, keep the car parked and have a mechanic come to you. Find a mobile mechanic in your city, or send a request with your year, make, model and engine, whether the light is steady or flashing, and any codes you already have.