Cadillac: 97 catera heat problem

Hi,
have a problem with a 97 catera I'm working on for a friend. Blowers are
running fine but no hot air at all. I've checked all the fuses and looked
for loose connections on the heater control valve but nothing. Engine
doesn't overheat at all. Just blows cold air.

Any thoughts on where to start checking?

thanks,
Chris

Have the same problem with 2002 buick Dod you find a solution to the
problem