A spark plug fires to begin the combustion process in an engine. It is connected via an electrical conduit between the battery and cylinder. A four-cylinder engine typically has four plugs, an eight-cylinder engine has eight plugs, and so on. Since a spark plug creates high-voltage electricity, it will wear out over time.

If your car won’t start or hesitates to start, idles poorly (feels shaky or bouncy), misfires, or there’s engine knocking or pinging, a spark plug may be failing. Declining fuel economy and vehicle performance are also indicators. If you notice any of these symptoms, follow our tips on how to change spark plugs so your vehicle runs normally again.

How Often to Change Spark Plugs

Replacement is generally recommended every 30,000 to 100,000 miles. Every vehicle model is different, so check with the manufacturer’s recommendations for when to change the spark plugs. You can wait until the car reaches a specific mileage. But if there are any changes in how your vehicle rides, have the sparks checked. They’re made of small copper, platinum, nickel, iridium, and ceramic parts that wear out; every time there’s a spark, small amounts of metal burn off, growing a gap until electrical sparks can no longer cross.

Steps to Changing Spark Plugs

It’s best to have a mechanic change the spark plugs, unless you have some experience with vehicle repair. To successfully change spark plugs, follow these steps:

  1. Create a Safe Work Environment: Park on a flat, dry surface and turn the vehicle off. Wait until the engine is cool to start working. Then clean away dirt and debris from the engine; blasting compressed air over the work area and the ignition coils can help. Disconnect the negative post of the vehicle battery to prevent electrical discharges and shocks.


  1. Disconnect the Spark Plug Wire: Before removing the wire, you may need to remove the upper intake plenum (which will require a new gasket), ignition coil, and/or rubber boot assembly. The boot may be difficult to remove; if damaged, it will need to be replaced. Use wire pliers to remove the metal terminal and boot together.


  1. Remove the Coil on Plug (COP): Disconnect the electrical connector from the ignition coil by pulling up or pressing down on the locking tab. Depress the tab with a small screwdriver if necessary. Once you’ve removed the connector, take out the hold down bolt and twist the coil about a quarter turn. If the bond isn’t broken, move it back and forth. You should then be able to pull the coil up and out.


  1. Unscrew the Spark Plug: Using a spark plug socket, remove the spark plug without damaging the threads. For COP configurations, blow air into the well hole to remove debris. Any particles can cause thread damage. To loosen the spark plug, twist it half a counter-clockwise turn and apply a small amount of penetrant fluid to the base. Wait a few minutes and try turning the spark plug again.


  1. Gap the Spark Plugs: If the spark plugs need to be gapped, slide a wire/gap gauge between the electrodes; there should be some slight drag in the wire. The gap can be widened by prying the tool up. Gaps that are too large can be narrowed by placing the plug on a solid surface and lightly tapping the side electrode.


  1. Install Your New Spark Plugs: Match the part number of the new spark plugs with that on the box, and inspect each part for damage to electrodes, threads, or the wire insulator. For pre-gapped sparks, check that the tip is not bent or damaged, while for other types, compare the gap with what matches the specifications for your engine. Tighten the spark plug without using too much torque, using a torque wrench.


  1. Re-Install the Spark Plug Wires/Ignition Coils: Add a small amount of grease to the wire or COP boot and connect each wire to the appropriate spark plug. Each spark plug fires for a specific engine cylinder, as shown in a firing order diagram. Re-install the coil hold-down bolts, re-attach the connectors, and re-install other components you may have removed. If the battery was disconnected earlier, re-connect the terminals so power will reach the spark plugs.


Now you can try starting the engine to see if it will run. Visit a mechanic if the Check Engine light is still on or flashes. This indicates a cylinder misfire, so you should avoid driving very far or you risk damage to the catalytic converter. To ensure consistent performance, it’s best to replace all spark plugs at the same time.


Contact Hawthorne Auto Square


We have hundreds of pre-owned, low-mileage luxury used cars, trucks, and SUVs that have been thoroughly inspected. A comprehensive 30-day/1,000 mile warranty is included with each purchase. Our dealership also covers engine, transmission, cooling system, brake, seat belt, inflatable restraint, catalytic converter, and electrical/computer component repairs. For more information on how to change a spark plug, get a service contract, and apply for stress-free financing, call 866-707-7664 today!