GMC 1995 Suburban Owner's Manual page 178

To Lock The System:
1. Write down any
number
from
000 to 999 and keep it in a safe place.
2. Turn the ignition to ACC or RUN.
3. Turn the radio off.
4. Press the 1 and 4 buttons together. Hold them down until
"-
-
-"
shows
on the display.
You are now ready to enter your secret code. Don't wait more than 15
seconds between any of the following steps. If you do, the radio
automatically returns to time display and you must start the procedure
over at step 4.
5. Press MIN and "000"
will appear on the display.
6. Press the MIN button again to make the last two digits agree with your
code.
7. Press HR to make
the first one or two digits agree with your code.
8. Press AM/FM after you have confirmed that the code matches the one
you wrote down. rEP appears in the display, indicating that you need to
repeat steps 5 through 7.
9. Press AM/FM and this time the display will show "SEC". "SEC"
means the system is secured.
With the ignition off, the THEFTLOCK indicator will flash.
To Unlock THEFTLOCK After a Power Loss
When battery power is reapplied to a secured radio, the radio won't turn on
and "LOC" will appear on the display.
Enter your secret code as follows. Pause no more than I5 seconds between
steps.
1. Turn the ignition on.
2. Turn the radio off.
3. Press MTN and "000" will appear on the display.
4. Press the MIN button again to make the last two digits agree with your
code.
5. Press HR to make the first one or two digits agree with your code.
6 . Press AM/FM after you have confirmed that the code matches the one
you wrote down. SEC appears in the display indicating that the radio is
now operable.
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