That is a security "feature" on GM radios to deter theft. The following instructions has had sucess with other GM vehicles. If not, only a dealer has the equipment to get past lock screen.
This works on vettes with theftlock. Try it on the tahoe and let us know....
The following procedure will give you a new code that you can use instead of your personal code. Using this new code, you can "unlock" your radio and then "re-lock" it with a new code if you wish. Prerequisites: The radio must be in LOC mode NOT in INOP mode. Let me explain. If you have activated the Theft-Deterrent feature of your radio and loose power, the radio goes
into LOC (theftlock) mode. With the ignition on, you must use the MN and HR buttons to enter you personal code in order to unlock it (The instructions are on page 3-27 in your Y2K Owner's manual). If you fail to enter the correct code eight (8) times, the radio goes into INOP
mode. You have to wait an hour with the ignition on before the radio returns to LOC mode (go for a long drive somewhere).
Procedure: 01. Turn the ignition ON (The radio displays LOC)
02. Hold down the Radio Presets 2 and 3 buttons for six seconds
03. The radio display changes from LOC to a three-digit number. Write this number down. Don't take forever, fifteen seconds max
before the next step.
04. Press the AM/FM button
05. The radio display changes to another three-digit number. Write this number to the right of the first one.
06. You now have a six-digit number, the first three digits are from STEP 03 and the last three digits are form STEP 05
07. Call 1 800 537 5140
08. Press 1 then # (pound). You'll hear "Invalid Code, try again"
09. Press 139010 then # (pound). You will be asked to enter your four or six digit code followed by * (start)
10. Enter the number from STEP 6 then * (star)
11. Listen to the four-digit number and write it down. It will be repeated twice
12. Turn the ignition ON (The radio displays LOC)
13. Use the MN and HR buttons to enter the code from STEP 11
14. Press the AM/FM button. The radio display changes to SEC Your radio is now "un-locked" and usable. The Theft-Deterrent feature is STILL active! If you want to disable it, use the instruction on page 3-27 in your Y2K Owner's manual. Use the code from STEP 11 instead of your personal code. Note: This tip originated from LOTZAHP, Austin TX (credit must be given where credit is due). I verified it on an Y2K C5 with a locked radio - mine.http://chevroletforum.com/forum/tahoe-suburban-25/help-radio-locked-8220/