The timer does not time out and so the fan runs on continuously

My fan is controlled from the light switch

My lighting is low voltage or low energy and has its own transformer

Ensure there is both a permanent live and switch live on the fan or transformer controller (where applicable) and that they have been connected to correct way round. See product fitting/wiring for full relevant wiring details.

Some types of low voltage and low energy lighting (mainly older types) can create a high voltage surge at the point the light is switched and this can interfer with and in some cases damage timers in fans. If your timer fan has failed due to lighting type you will likely need to replace it. If placing on the same lighting circuit, your electricial should protect the fan with a supression device such as a Varistor of type relevant to the lighting type.