Turn the ignition on (dash lights on, dont start engine), press and hold the lock button on the fob for 5 seconds, turn ignition off, remove key, wait ten seconds, remote should now work.
Have you checked/renewed the fob battery?
If that doesn't work, take the key to a decent garage/auto electrician/locksmith and ask them to test it. They just point it at a receiver like this
http://www.trueshopping.co.uk/product/Draper_Expert_Key_Fob_And_Remote_Control_Tester/3492/46314.html
and push the button, so it shouldn't cost much ( I just bought my own tester).
geo, March 2010
