Chevrolet 2004 Avalanche Owner's Manual page 79

To replace the battery in the keyless entry transmitter,
do the following:
1. Insert a thin object, such as a coin, in the slot
between the covers of the transmitter housing near
the key ring hole. Remove the bottom by twisting
the coin.
2. Remove and replace the battery with a
three-volt CR2032 or equivalent battery, positive (+)
side up.
3. Align the covers and snap them together.
4. Resynchronize the transmitter. See
"Resynchronization" next.
5. Check the operation of the transmitter.
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
15 seconds. The door locks should cycle to confirm
synchronization. If the locks do not cycle, see your
dealer for service.
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