If your car won't start then it could be due to your vehicle's immobiliser system. The following is the best way to isolate the problem:

Check the VERY obvious but sometimes overlooked first - Is there petrol in the vehicle? Is the battery flat? Are the connections to the battery loose?

If the engine occasionally splutters as if it is just about to start but doesn't fire up fully, starts and keeps cutting out or turns over very slowly then this is likely not related to the immobilser.

If the engine won't turn over at all (no sound when turning key) or the starter motor just continues to turn but the engine won't fire you may have immobilser issues. Many modern vehicles do have an immobiliser warning light on the dashboard (usually a sillhouette of a key) - check your manual. But some vehicles will not have this warning light. And if the immobiliser is not factory fitted then the light will not come on.

Do the following:

- Lock up the vehicle using the remote if equipped and open again. This may reset the immobiliser. Then try and start the vehicle

- Check your vehicle's owner's manual and look up fuel cut off resetting. Many vehicles have a safety system which cuts off fuel supply in the event of an impact. This can sometimes cut in inadvertently. Normal procedure is to press a button situated either under the bonnet (hood) or under the dashboard.

- Next, find any spare keys and try them. This will rule out any problems with the transponderchip within your key. Although this does not rule out problems with the transponder aerial.

_ Disconnect the vehicle's battery for 5 to 10 minutes, reconnect and then try starting the vehicle. Again this may reset the immobiliser/ECU.

If all this fails then further investigation will be required by a specialist.

Immobilsers work in several ways:

- disconnect voltage to HT leads and subsequently stop spark plugs from firing

- disconnect voltage from fuel pump

- disconnect voltage from starter motor

Depending on the vehicle the immobiliser system may employ any combination or all of the above.

If the starter motor is turning then the usual course of action an engineer would take is to see if the plugs are sparking or that fuel is being pumped and then work back to the cause of any irregularity.

Of course the problem could be totally unrelated to the immobiliser and be a faulty fuel pump or ignition system or just a wire unplugged but most engineers would still go about diagnosing the fault in this way. Alternatives are diagnostic fault code reading systems which an engineer would plug into the vehicle's diagnostic socket with specialist equipment and then be able to check all systems in the vehicle as well as any fault codes that may have been logged. These diagnostic systems can make fault tracing easier.