Spark plug replacement: what to know

What the job involves, what changes the price, and what to ask a pro.

What spark plug replacement is

A technician removes the old spark plugs, checks the ignition parts, and installs new plugs with the correct gap and torque. You need it when the engine misfires, runs rough, or has reached the plug interval listed in the owner's manual.

Signs you need it

  • The engine misfires, shakes or runs rough at idle
  • Fuel economy has dropped without another clear cause
  • The check engine light shows a misfire code
  • The engine is slow to start or hesitates under load
  • The plugs are past the interval in your owner's manual

What the pro does

  • Reads codes and confirms which cylinders are misfiring
  • Removes the old plugs and inspects their tips for fouling, wear or oil
  • Checks the ignition coils, wires or boots for cracks and tracking
  • Installs new plugs with the gap and torque the engine requires
  • Road tests the car and clears codes to confirm smooth running

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Engine layout and how many plugs the engine has
  • How hard the plugs are to reach, including whether parts must be removed first
  • The plug type required, such as standard, iridium or platinum
  • Whether ignition coils or wires also need replacing
  • Whether the engine has a plug that is seized or damaged on removal
  • Whether the job is part of a tune up that includes other service

What a written quote should spell out

  • The plug type, brand and part number for each cylinder
  • Whether the plugs were gapped or came pre gapped
  • Torque specification used during installation
  • Any ignition coil or boot replacement, listed separately
  • Warranty for the plugs and the labor

Questions to ask before you hire

  • Which plug type does my engine require?
  • How hard are the plugs to reach on my engine?
  • Did you check the coils and boots as well as the plugs?
  • Will you check for damaged threads when the old plugs come out?
  • Do you use the torque specification from the manufacturer?
  • Can I see the old plugs after the job?

Red flags

  • Recommends a plug type that differs from the one listed for your engine without a reason
  • Says the plugs are fouled without showing them to you
  • Will not give the torque specification or the gap used
  • Adds coils or wires to the quote without test results

What you can do yourself

Replacing spark plugs is a common job on some engines, but many engines have plugs that are hard to reach and can be damaged by over tightening. Only attempt it if you have the correct plug type, the gap and torque specification, and a torque wrench. Call a shop if the plugs are hard to reach, the engine is still warm, or the threads look damaged.

Licenses and permits

A license is usually not required for spark plug replacement. Auto repair rules differ by state and city, so check with your state or local licensing agency if you are unsure about a shop.

Spark plug replacement questions

How often do spark plugs need replacing?

Use the interval in your owner's manual. The plug type matters, because some plugs are designed to last much longer than others.

Can bad plugs damage the engine?

A misfire left alone can damage the catalytic converter and waste fuel. Have the misfire checked when the light appears.

Why do the new plugs need a torque check?

Plugs that are too loose can leak combustion gas, and plugs that are too tight can damage the threads in the head.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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