On my '01 MC SS (3.8L), the cooling fans constantly run while the engine is running and a few minutes after the key is turned off. The temperature gauge is reading the lowest it can go. I have replaced the thermostat and the coolant temp sensor. The thermostat doesn't open and the car does have heat. I am very confused on what is wrong. Any help is appreciated.

 

So are the fans roaring or just running?
to check the thermostat to see if it's working or not, start the thing up with the radiator cap off, when it warms up enough it will show circulation (old school way).
Also the system after the Thermostat replacement needs to be bled out. if there is a bubble in it, it can overheat.
It could be the intake manifold gaskets are bad and they need to be replaced. This will cause the overheating issue. Check out the list of youtube videos in this link.
https://www.google.com/search?source...60.Srw4pCWcHjs

 

Last night after I installed the thermostat, I let it run for about 35 minutes with the bleeder valve open to bleed the system and the thermostat didn't open then. I could squeeze the upper radiator hose. The fans were running like the engine needed to be cooled down.

As of this morning; I let the car warm up as normal for about 20 minutes (it's like 20°F here in Minnesota) and then started going to school. On my way to school, the temp guage went up to the normal operating temperature and fluctuated between 200 and 190ish like normal. I couldn't tell if the fans were still roaring or not. I'll know more later today. I am going to bleed the system later on today as well.