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All car petrol engines use spark plugs to ignite the fuel-air mixture in the cylinders, but these plugs gradually wear out and have to be replaced. How often depends on your manufacturer - take a look at your vehicle handbook or Haynes manual for details.


  • New set of compatible spark plugs
  • Spark plug socket spanner
  • Torque wrench
  • Short length of flexible hose
  • Copper grease
  • Set of feeler blades or a spark plug gap gauge (if needed)

Step 1 - Preparation

Open the bonnet and remove the HT leads and any obstacles in order to access the spark plugs - see your vehicle handbook or Haynes Manual for more info.

Step 2 - Clearing the Area

Brush or blow away any debris from around the spark plugs, then unscrew them with a spark plug socket spanner and remove them.

Step 3 - Checking the Plugs

Use the feeler blades or the gap setting tool to check the electrode gap of the new plugs, if applicable - see your vehicle handbook or Haynes manual.

Step 4 - Preparing the Plugs

Put a smear of copper grease on the threads of each new plug and screw them into their holes. Use a length of flexible hose to start them off.

Step 5 - Tightening the Plugs

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

Reconnect the HT leads and refit any other parts that have been removed.

Haynes Hints

  • Fit new HT 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.