Buick Century 2003 Owner's Manual page 79

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.
To replace the battery do the following:
1. Insert a flat object like a thin coin into the slot on
the back of the transmitter. Gently pry apart the
front and back.
2. Remove the old battery and replace it with the new
one. Do not use a metal object to do this. Use
type CR2032 or an equivalent. Make sure the
positive (+) side of the battery is facing down.
3. Snap the top and bottom together, making sure the
halves are together tightly so water won't get in.
4. Resynchronize and test the operation of the
transmitter with your vehicle.
Resynchronization
After you have changed the battery in your transmitter,
you will need to resynchronize the transmitter. To do this,
press the LOCK and UNLOCK buttons at the same time
and hold for about seven seconds or until one horn chirp
is heard.
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