Here’s how to replace those spark plugs yourself.  

The car shown has easily accessible plugs. Not all models will be as simple as this and not all will have HT leads. For details applicable to your car, see the relevant Haynes Manual.

You will need a set of spark plugs of the correct type for your car, a spark plug socket and a torque wrench, plus a set of feeler blades or a special spark plug gap setting tool. A short length of flexible hose and some copper grease will also come in handy.

Be warned - switch off the ignition before starting work. Do not attempt to remove spark plugs while the engine is hot - not only may you burn yourself, you could damage the cylinder head.

1. Open the bonnet and remove whatever parts you need to in order to get at the spark plugs and HT (high tension - high voltage) leads. Identify the leads if necessary (they may already be numbered) and disconnect them from the spark plugs by pulling on the connector, not on the lead.
2. Brush or blow away any dirt from around the spark plugs, then unscrew them with the socket and remove them. Have a look at the firing end of each plug: it should be a light brown or grey colour. A spark plug which is black and sooty, or oily, may be a warning of a problem developing.
3. Use the feeler blades (3a) or the gap setting tool (3b) to check the electrode gap of the new plugs (see your Haynes manual to find out what the gap should be). Adjust if necessary by carefully bending the side electrode. Some plugs don’t have an adjustable gap, in which case skip this step.
4. 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 - if you get a spark-plug cross-threaded, the hose will slip before you cause any damage.
5. Final tightening of the spark plugs should be done using a torque wrench, otherwise there’s a risk of doing them up either too tight or not tight enough. See your Haynes manual for the torque setting specified for your engine.
6. Reconnect the HT leads, refit any other components and start the engine. If it won’t start, the chances are you’ve connected the HT leads in the wrong order.