Chevrolet 2007 Express 2007 Express Owner's Manual page 106

Resynchronization
Resynchronization may be necessary due to the
security method used by this system. The
transmitter does not send the same signal twice to
the receiver. The receiver will not respond to a
signal that has been sent previously. This prevents
anyone from recording and playing back the
signal from the transmitter.
To resynchronize your transmitter, stand close to
your vehicle and press and hold the lock and
unlock buttons on the transmitter at the same time
for seven seconds. The door locks should cycle
to confirm synchronization. If the locks do not
cycle, see your dealer for service.
106
Battery Replacement
Under normal use, the battery in your remote
keyless entry transmitter should last about
four years.
You can tell the battery is weak if the transmitter
won't work at the normal range in any location.
If you have to get close to your vehicle before the
transmitter works, it's probably time to change
the battery.
Notice: When replacing the battery, use care
not to touch any of the circuitry. Static
from your body transferred to these surfaces
may damage the transmitter.