GMC 2003 Sierra 1500 Pickup Owner's Manual page 244

When a traffic announcement comes on the current
station or on a related network station, you will hear it,
even if the volume is muted or a compact disc is
playing. The traffic symbol and TRAF will appear on the
display while the announcement plays. If the compact
disc player was being used, play will stop during
the announcement.
For DAB (Canada only, if your radio is equipped with
DAB), when the TRAF button is pressed, DAB does not
seek to a station that broadcasts traffic. DAB only
checks the current frequency for traffic support.
This function does not apply to XMTM Satellite Radio
Service.
Traffic Interrupt Feature: Your radio can interrupt the
play of a cassette, CD, or XMTM, or DAB station.
Press the TRAF button. The radio will seek to a station
that broadcasts traffic announcements. When the
radio finds a station that broadcasts traffic
announcements, it will stop. TRAF will appear on the
display. When a traffic announcement comes on
the station that was found, you will hear
it.
When the
traffic announcement is over, the radio will resume play
of the cassette, CD, or XMTM, or DAB station. If no
station is found, NO TRAF will appear on the display.
Radio Messages
CAL ERR (Calibration Error): This message is
displayed when the radio has not been calibrated
properly for the vehicle. You must return to the
dealership for service.
LOCKED: This message is displayed when the
THEFTLOCK@ system has locked up. You must return
to the dealership for service.
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