There is no evidence in the Corvette owners manual either,,,,
Here is how it is done in a 2001 C5 Corvette
How to Enter Diagnostic Mode
The "diagnostic display" mode is entered with the following procedure:
1) Turn on the ignition but don't start the engine.
2) Press the "reset" button to turn off any warning messages
3) Press and hold "options"
4) While holding "options", press "fuel" four times within a 10-second period.
When first engaged the on-board diagnostics go into an "automatic" mode which shows all parameters: PCM-TCS-RTD-BCM-IPC-radio-HVAC-LDCM-RDCM-SCM-RFA. If there are no problems, you will see "no more codes" on the ICP display.
The computer will show two kinds of codes, "current" and "history," designated "C" or "H". A current code indicates a malfunction is present in the system whose module is displaying data. A history code indicates a past problem.
When the screen displays "manual diagnostics," select the desired module by pressing the "options" button to go forward or the "trip" button to go back. Press "gages" to go forward or "fuel" to go back.
To exit the diagnostic mode at any time, press "E/M". If you want to erase or "clear" codes, press " reset
Do you think there is also a secret code for the SKY
For the 1999 Pontiac Firehawk you can use the OBDII module or others to plug into the input-port underneath the steering wheel. I've noticed the Sky has that same port.
Another way to tell on the Firehawk was to do a series of on/off clicks with the ignition key and OEM radio buttons which would give a series of door chimes in ones and then tens and hundreds. From that, there is a chart you can diagnose your issue that is in the shop manual (not the owner's manual).
I had a faulty catalytic converter O2 sensor (Code 0430) that got fixed by that method.
Call OnStar they can do the diagnostic and tell you what is wrong with the car if something is wrong. Just reach up and push that middle button and say the car is running rough and making a funny noise, you don't even have to turn the car on they will link up with the on board computer. If a code shows up they will let you know, not sure they know what all the codes stand for so if you do this, let us know what they say!