Cadillac: On board diagnostics for heater problems, dick dick, air turbulence

Question
1998 Cadillac DeVille with inadequate interior heating. Has been working fine for 49,000 miles. Air turbulence sounds like fan is running but an airmix door is not opening.The air conditioning/heater programmer has been replaced by a Cadillac dealer shop. Does this model have a plug for on-board diagnostics of the problem displaying Trouble Codes? I have been told that this was not available before the 1999 models.

Thank you for any help you can give,
Dick

Answer
Dick-I am not a Caddy technician, but I do remember a friend taking a car in for diag (for trouble codes with regard to air conditioning malfunction)
Go to your local caddy dealer, walk in back like you own the place and talk to a tech and get the real skinny.
Ive done this on occasion with my Ford, which I am not well versed with..(and it always seems to need something as well)
Good luck!
Brian
Superior Auto Institute
www.nodents.com