The slug fence can have the following three statuses:
The green LED pulsates regularly every four to five seconds.
The fence is working correctly.
The red LED pulsates once every four to five seconds.
This may be the result of a tear somewhere along the aluminium strips. Find and clean the damaged section, and tape over it with a short strip of aluminium tape.
It may also be the result of a circuit fault. The slugs leave behind a trail of slime that conducts the flow of electricity. If there are several conductivity spots between the aluminium strips (which usually turn green after a certain length of time), the impulses can become weaker, as signalled by the flashing red LED. For this reason, make sure that you wipe the fence with a soft damp cloth or paper towel occasionally.
The red LED pulsates briefly twice every four to five seconds.
The battery may be low on power. Replace the battery. However, normally one battery should be sufficient for one garden season.