Spark plug replacement is not a particularly complicated service, but it can be quite difficult on some engines. That is because access to the spark plugs is often limited and requires special tools. And if a spark plug breaks before it comes loose from the cylinder, the repair can be significant.

The specific process for replacing spark plugs also differs depending on the age of your vehicle and the type of ignition coil(s) used. Older models featured a distributor that controlled the way each spark plug was fired, the timing between cylinders. On those engines, spark plug wires extended from the distributor cap to each spark plug. Some newer models with electronic ignition rely instead on a set of coils that get their direction from the engine control module. Spark plug wires, in those cases, run from the coils to the spark plugs. The latest technology includes coils that are installed directly on top of each cylinder and spark plug in an arrangement known as “coil-on-plug” or COP.

To remove and replace the spark plugs, a technician must do the following:

• Disconnect the negative terminal on the battery, taking care to preserve the computer memory in the vehicle
• Unplug the ends of the spark plug wires from each spark plug or unplug the coil connectors (in the case of COP) and unbolt and dislodge the coils from the plugs
• Clean out the holes where the spark plugs are mounted so that debris does not enter the cylinders
• Using a special socket, unscrew the spark plugs and extract them from their mounts
• Using a feeler gauge, check the gaps between the two electrodes on each spark plug to ensure that they are set to manufacturer’s recommendations and adjust if necessary
• Insert the new spark plugs and torque to manufacturer’s specifications
• Reinstall the spark plug wires or coils and connectors
• Reconnect the battery
• It is common to also replace either the spark plug and coil wires (older models) or the coil boots (newer models) when replacing a set of spark plugs.