Spark Plug Replacement: Signs, Intervals and What It Costs

What is spark plug replacement, and what do the plugs do?

Spark plugs sit in the top of each cylinder. AAA explains that they create the electrical spark that ignites the fuel and air in the cylinders, which is what produces the engine’s power. The spark itself comes from the ignition coils, so plugs and coils are usually checked together.

Replacing them means removing the old plugs, checking what they look like, and fitting new plugs of the type and gap your engine calls for. The job is often sold as a “tune-up.” According to AAA, a modern tune-up can also include replacing filters, checking the ignition system, topping up fluids and inspecting belts and hoses, so ask exactly what a quoted tune-up covers.

What are the signs you need new spark plugs?

AAA lists these common signs of worn or failing plugs:

• Hard starting, especially in cold weather.
• Rough idle or vibration while the car is stopped.
• Misfires that turn on the check engine light.
• Worse fuel economy than you are used to.
• Sluggish acceleration or hesitation when you press the gas.
• Unusual engine noises, such as rattling or popping.

Plugs are not the only cause of these symptoms. A failing coil, a fuel problem or a vacuum leak can feel the same from the driver’s seat. AAA Club Alliance notes that failing spark plugs or ignition coils can reduce engine power and fuel economy, and that ignoring worn plugs long enough can damage the catalytic converter, which is a much bigger repair.

How long do spark plugs last?

It depends mostly on the plug material. These are the typical ranges AAA gives:

Your owner’s manual has the interval for your engine, and that is the number to follow. AAA Club Alliance says plugs in vehicles built this century can last up to 100,000 miles, but they don’t always. If you bought the car used and have no record of the last change, a mechanic can pull one plug and look.

Can a mobile mechanic replace spark plugs in the driveway?

Yes, on most cars. Spark plug replacement needs hand tools, a torque wrench and the right plugs, not a lift, so it is a routine mobile job. The mechanic works from the top of the engine, and the car usually doesn’t need to be raised at all.

How long it takes depends on the engine. On some engines the plugs are right under the coils. On others, other parts have to come off first to reach the rear plugs, which adds time and labor. Give the exact year, make, model and engine size when you ask for a quote.

Where a driveway visit reaches its limit: if the misfire turns out to come from inside the engine rather than the ignition parts, internal engine work is shop work. A mechanic may also suggest a diagnostic visit first if the cause of a misfire isn’t clear. See what a mobile mechanic can and can’t do for the general limits.

What does the job involve?

1. Codes and symptoms. The mechanic reads any stored codes with an OBD-II scanner. A code for a single cylinder can point to one plug or one coil.
2. Access. With the engine cool, covers and coils come off, along with any parts in the way.
3. Removal and inspection. Each old plug comes out and is checked. Oil, soot or damage on a plug can point to other problems worth mentioning to you.
4. Gap and install. New plugs are checked for the correct gap and threaded in carefully, then tightened to the specified torque. AAA warns that poor installation can cross-thread a plug, damage the cylinder head if overtightened, leak combustion gas if undertightened, or cause misfires if the gap is wrong.
5. Reassembly and test. Coils and covers go back on, codes are cleared, and the engine is started and test-driven.

If a coil is cracked or tests bad, it can be replaced in the same visit. See ignition coil replacement.

How much does spark plug replacement cost?

These ranges come from the prices that mechanics we list publish on their own websites (October 2026). Prices vary by vehicle and city, and the mechanic quotes after looking.

Engines with more cylinders, harder-to-reach plugs or premium plug types sit toward the top of the range. Coils, if needed, are usually quoted separately. See mobile mechanic cost for the full price table.

What should you ask before booking?

• Is the quote for plugs only, or a full tune-up? What exactly is included?
• What plug type are you fitting, and does it match the owner’s manual?
• Are the parts OEM or aftermarket?
• If a coil or boot is bad, will you call me with a price before replacing it?
• Will you show me the old plugs?
• Is there a trip fee, and is it credited if the work is done today?

A mechanic who can see the car will know more than any guide.

What to do next

If you are past the plug interval in your owner’s manual, or the car is misfiring, idling rough or starting hard, have it looked at before the problem spreads to other parts. Find a mobile mechanic in your city, or send a request with your year, make, model, engine and mileage. If the check engine light is on, our check engine light guide explains what to note down first.