I might be talking bollocks here as I know fuck all about cars
but we had problems with our zapper not working and we'd changed the battery, it worked for a few hours and then stopped again.
Did this about 3 more times before the guy at the car shop asked to look at the set of keys that lived with the car keys on the same chain.

Turns out it was the magnetic trolly keyring that was attaching itself to the car key and draining the battery straight away.
( Mrs Sp@m, Thu 20 Oct 2011, 14:30, archived)

That's very useful advice. Thank you.
Snot that though. Nothing funny on my keyring, and I've tried the batteries.
I've a suspicion that it's condensation inside the sensor thing. But it might not be. If it's something more serious, the day's coming when I won't be able to start it. What a bother. And it probably isn't worth paying to get it fixed either.
(PERSONALITY HORSE Graham Tapes - free download album - coming soon, Thu 20 Oct 2011, 14:35, archived)