GMC 2006 Sierra - Manual (Page 283)

BAND (Alternate Frequency): Alternate frequency
allows the radio to switch to a stronger station with the
same program type. To turn alternate frequency on, press
and hold BAND for two seconds. AF ON will appear on
the display. The radio may switch to stations with a
stronger frequency.
To turn alternate frequency off, press and hold BAND
again for two seconds. AF OFF will appear on the
display. The radio will not switch to other stations.
This function does not apply for XM™ Satellite Radio
Service.
Setting Preset PTYs (RDS Only)
These pushbuttons have factory PTY presets. Up to
12 PTYs (six FM1 and six FM2), can be programmed
on the six numbered pushbuttons, by performing
the following steps:
1. Press BAND to select FM1 or FM2.
2. Press the TYPE button to activate program type
select mode. P-TYPE and the last selected PTY
will appear on the display.
3. Turn the TYPE knob or press and release the
TYPE button to select a PTY.
4. Press and hold one of the six numbered
pushbuttons until you hear a beep. Whenever that
numbered pushbutton is pressed, while in PTY
mode, the PTY that was set will return.
5. Repeat the steps for each pushbutton.
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