1998 Cadillac Catera Heater blows out cold air

You have a heater control valve that is in the rear of the engine, and somewhat difficult to access. There is a vacuum line going to it. You need to determine if vacuum is getting to that valve. I am not sure if it needs vacuum to let hot water flow to the core, or if no vacuum allows it to flow. It could also have a faulty diaphragm, and therefore be non-functional.

Did you figure out problem. I too have the same problem. I am told that the dash needs to come out and the acuaitor (sp?) maybe bad. I hate this car.