How spark plugs work

When an engine's piston (a moving part in the combustion chamber) compresses the air-fuel mixture in the cylinder, a spark plug delivers a precisely timed electric spark. This spark ignites the mixture, causing a controlled explosion. The explosion generates a powerful force that drives the piston down and converts chemical energy into mechanical energy, which powers your car and gets it moving.

Replacing spark plugs

When to replace your spark plugs depends on a few factors, including the type of spark plugs your vehicle uses, spark plug wear, the conditions you drive in, and your manufacturer’s recommendations. Most modern spark plugs are designed to last for a significant mileage range, typically between 30,000 and 50,000 miles.

HOW TO FIT NEW SPARK PLUGS

All car petrol engines use spark plugs to ignite the fuel-air mixture in the cylinders, which puts the pistons in motion and gets your vehicle moving.

Over time, these plugs will gradually wear out and have to be replaced. How often you’ll need to change them will vary by make and model, but you can find this information in your vehicle handbook manual.

With the right tools, you can change your spark plugs yourself. Here’s our step-by-step guide on how do it safely:

Checklist

To fit new spark plugs, there are a few items you’ll need to have to hand:

New set of compatible spark plugs

Spark plug socket spanner

An extender (if needed)

Torque wrench

Set of feeler blades or a spark plug gap gauge (if needed)

Step 1 - Preparation

Locate your spark plugs under the bonnet using your owner’s manual. They will usually be either on the top or side of the engine. Before attempting any work, remove any loose dirt and debris from around the spark plugs, and remove any obstacles that prevent access to the spark plugs.

Top tip: always wait until your engine is cool before beginning any work.

Step 2 - Remove Ignition Lead

Once you’ve located the spark plugs, remove the first ignition lead (also known as a high-tension lead, or HT lead). These are thick, insulated wires attached to the spark plugs.

We recommend only removing these one at a time as you change each plug, to avoid mixing up which lead attaches to which spark plug.

Step 3 - Remove Old Spark Plug

With the ignition lead removed, use your spark plug socket spanner (with an extension attached if required) to unscrew the spark plug from the well.

Step 4 - Prepare the New Plug

You’re now ready to install the new plug, First, use the feeler blades or the gap setting tool to check the electrode gap. This gap must be a set distance, listed in your vehicle handbook or Haynes manual.

Once the gap is correct, put a smear of copper grease on the threads of the new plug and screw it into the well.

Step 5 - Tighten the Plug

Final tightening of the spark plugs should be done using a torque wrench. See your vehicle handbook or Haynes manual for the correct torque.

Step 6 - Finishing Off

With the new spark plug fitted, you can reconnect the ignition lead, and repeat this process for each spark plug.

Once all your spark plugs have been changed, refit any other parts that you removed in order to access the plugs.

Helpful Tips:

Fit new ignition leads if the old ones are cracked, oily or otherwise damaged.

Cleaning spark plugs with a wire brush or an abrasive cleaner is no longer recommended because of the risk of damaging them.

Black, sooty spark plugs can be a symptom of a clogged air filter element.