GMC TopKick 2007 Owner's Manual page 88

3. Align the covers and snap them together.
4. Check the operation of the transmitter. If the
transmitter does not work after battery
replacement, it may need to be
resynchronized to your vehicle. See
Resynchronization following.
88
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 it 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 simultaneously press and
hold the LOCK and UNLOCK buttons on the
transmitter for at least five seconds. The door locks
should cycle to confirm resynchronization. If the
locks do not cycle, see your dealer for service.