GMC 1999 Sierra 1500 Pickup Owner's Manual page 145

on the driver's door to show the old mileage reading when the new odometer
was installed.
Trip Odometer
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K2323
The trip odometer can tell you how far your vehicle has been driven since
you last set the trip odometer to zero.
To reset the trip odometer, fully press the reset button located near the
readout. If the reset button is not fully pressed, the trip odometer may not go
all the way back to zero. If it doesn't, you may have to press the reset
button again to reset the readout to zero.
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