Cadillac Repair: Heater, seville sls, diagnostic center

Question
I have a 97 Seville SLS, the engine heats up but blows out cold air. can mess with the controls and then it will start getting warm inside the cabin, AC works fine but a little late in the season for it.is there a valve or gate that I could check.or what kind of code should I be looking for in the onboard diagnostic center?
Thanks Steve!

Answer
IS or HAS the CHECK ENGINE light ever illuminated while driving the vehicle and or has any other lights illuminated or any messages from the message display center ?.

What work or diagnostics of ANYKIND WHAT-SO-EVER have been performed on the vehicle trying to fix this problem ?.