When it comes to using fuel efficiently, spark plugs are one of the most critical parts of a petrol car's engine. And they operate in the most hostile environment.

Thousands of times per minute, they must spark strongly in each hot and highly pressurised engine cylinder. It's not surprising that they wear out.

But they do so gradually - over tens of thousands of miles. So, you may not notice the resulting deterioration in performance - the increased fuel consumption and decreased acceleration.

Eventually, the signs become more noticeable - misfiring, erratic running and difficult starting.

Long before that, the spark plugs should have been replaced. The replacement interval varies depending on the manufacturer; anything from every 10,000 miles to 60,000 miles or more may be specified.

If this sounds like a job for a garage at £60 or more per hour, don't be put off.

Replacing spark plugs is relatively straightforward provided you approach the task in a methodical manner. Here's how to do the job yourself.

The car shown has easily accessible plugs. Not all models will be as simple as this and not all will have HT (high tension - high voltage) leads. For details applicable to your car, see your manual. If you own a diesel car, you need read no further diesel fuel doesn't require spark plugs to ignite.

You will need a set of spark plugs of the correct type for your car (there are a large number of different ones), 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 leads. Identify the leads if necessary (they may already be numbered) and disconnect them from the spark plugs by pulling on the connector head, not on the lead itself.

2 Brush or blow away any dirt from around the spark plugs, then unscrew them with the socket and remove them. It's worth paying a little extra for a decent socket as some very cheap imports are of poor quality and could cause damage. 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 no adjustment is necessary.

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.

TOP TIP: Cleaning spark plugs with a wire brush or an abrasive cleaner is no longer recommended. It can do more harm than good by leaving conductive deposits on the plug's insulating nose. Modern ignition systems work at very high voltages and the HT current can leak away via deposits left by cleaning.