Spark plug replacement

How much does it cost to replace spark plugs?

How much could I save by replacing my own spark plugs?

Spark plug replacement is a straightforward job, although it can involve having to remove other parts to be able to gain access to the spark plugs. Depending on the number of spark plugs involved, this job should take a home mechanic less than a couple of hours. A main dealer can charge you up to around £150 an hour's labour to fit new spark plugs, so this is a job well worth doing yourself, with a Haynes Manual by your side.

Find a manual for your car here and start saving with Haynes

How spark plugs work

Spark plugs are used in petrol engines as a source of ignition at the end of the compression cycle. The spark ignites the petrol, which forces the piston along the cylinder, which in turn forces the crankshaft to rotate. Without the spark plug the engine wouldn’t be able to run. Diesel engines use glow plugs.

Most modern spark plugs are made from ceramic and iridium or platinum and should last for 60,000 miles plus. However they should be inspected every 30,000 miles or so, which will give you an idea of the health of the engine.

This task requires little experience, uses basic tools, and will take up to an hour, depending on your model.

When spark plugs need to be replaced

Consult your car’s manual, which should list the recommended intervals for changing your spark plugs. If you can’t find a recommendation, it’s worth removing them and checking their condition. It doesn’t take long and also gives you a clue as to how the engine is performing.

What should a healthy spark plug look like? It will have light brown electrodes and insulator, and no sign of melting, wear or deposits. However if you see oil deposits the piston rings may be worn. White deposits could mean that the plug isn’t of the right heat grade and carbon deposits show a rich mixture or weak ignition.

All cars are slightly different, so if it is time to change your spark plugs, use our before you begin checklist, and find your car for specific instructions.

Why you should change your spark plugs

Spark plugs need to be maintained properly if the engine is to operate smoothly. Neglect them and you could experience several issues.

First, you may find it difficult to start the engine. You’ll have to turn it over several times before it fires and this may flatten the battery over time. Once the engine is running, it may have a rough idle and feel ‘lumpy’ or it may misfire, which indicates that not all cylinders are working as they should. Misfiring should be sorted immediately, because you risk damage to the catalytic converter, which is expensive to replace.

A faulty spark plug may also cause increased fuel consumption and a reduction in engine power, and should be investigated at your earliest opportunity.

Tools you will need

Only basic tools are required for this job, although you may need to remove other components to access the plugs.

• Spark plug socket
• Torque wrench
• Ratchet
• Extension
• Spark plug gap gauge

Parts you may need

• Spark plugs - Refer to your handbook for the correct specification
• Spark plug wires/HT leads - You may need these if the old ones are in poor condition