Heat setting wont run on auto,its stuck in the economy mode

did you use the correct type of Freon for the compressor type?

Unfortunately, it sounds like your HVAC control module is failing. You can try a reset by removing the negative battery lead (under the rear seat) for about 10 minutes, and then reconnecting it, but if that doesn't work, you will probably have to replace the module.

If when you push auto it goes to econ that normally means you are low on freon.

your ac compressor is not coming on, could be low freon, or bad compressor,

If the a/c does not engage as a result of the electronic control system in your 1996 Cadillac, the climate control system may need to be replaced.

However, if the problem is being caused by low charge of R134a in the system, have the system charged before changing that electronic part.

I have never seen a Cadillac Climate Control System do this, which leads me to believe that it may be more electronic than refrigerant pressure related.

If the refrigerant pressure was low in the system, the climate control system should still stay on 'auto' mode, I think, and simply blow warm air.

Alot of the time the cooling system will only work in Economy when your cooling system needs to be recharge. It does this to protect AC pump. Recharge and check for leaks. Should solve your problem.

i have a 96 DeVille that did the same thing. It means your coolant tank is almost empty. It switches automaticlly to preserve based upon temp readings.

change the thermostat

I would recommend a salvage yard to get a good used one alot cheaper also try this place they carry only GM parts: Schram Auto and truck parts 1-800-292-1032 and also explain to them the problem you are having and these guys can assist you in getting the right part

Firing order 1-2-7-3-4-5-6-8 ..............1-3-5-7 back of engine near fire wall...........2-4-6-8 front of near radiator