My '98 STS has the dual climate system, with dials for both zones temperature controls.

How do you access the DTC codes wit this system?

I was told "OFF" and "PASS WARMER", but there is no "PASS WARMER" Button, it is a free turning dial to adjust temp up or down.

 

To check for Diagnostic Trouble Codes stored in the computer, turn the ignition on, then hold down the OFF and WARMER (for digital dash) or OFF and passenger-WARMER buttons (for analog dash) or OFF and "Info ^" buttons (if you have temperature dials) until all of the dash lights come on. The digital display will then show any codes stored. Be prepared to write them down. When it's done, you will probably see "PCM?" At that point, press the OFF button 3 or 4 times until the dash goes back to normal, or just turn off the ignition.

If that doesn't work for some reason or you need more explanation or information, check here:

http://myweb.accessus.net/~090/cadidiag.html

 

The problem with now knowing how to access codes is you wont have the factory manual to troubleshoot the problem. You will have the bullets but not know how to fire the rifle! Best tool you can have is a factory manual assuming you want to, at least, diagnose the problem.