Thermostat?

You're going to do the job yourself?
It's pretty easy.

Thermostat, gasket, and antifreeze should be about 20 25 buck

Hello

Several things may be the problem. If it has only been the last couple of years then obviously something changed.

Things to check for insufficient heating are, a leak in the cooling system, a low coolant level, a loose or worn drive belt, a leaking or kinked heater hose, a cracked or missing radiator cap seal, a stuck thermostat.

My suggestion is your vehicle calls for a 180 degree thermostat. Your vehicle states the thermostat should begin to open about 188 degrees and be fully open at 206 degrees. You may have a thermostat that is stuck open therefore never allowing the vehicle to properly warm up.

I would inspect the heater hoses really good. See if they are hot. If one is and one isn't then your heater core is plugged.

I would change my thermostat to the 180 degree, you can even go to a 195 degree.

I would also replace my radiator cap.

Your vehicle does have a Heater Core Temperature Sensor Switch but I can not find any information on it... It's location or what it does.

Hope this helps.

2000 Blazer with a 4.3 should have a 195 degree thermostat. The 4.3 from 1994 up all use a 195. As do 95% of the current vehicles on the road. The 160 is coasting you gas mileage as well as heat. At 160 the computer thinks the engine is still cold and feeds more fuel to the engine.

Try replacing the thermostat and also test the coolant temperature sensor-

It shouldn't be bouncing around but 200 is not overheating. It may have some air pockets in the system too from a slow leak.

Maybe thermostat sticking would be good to replace and flush and fill. Also check fan clutch when vehicle at op temp fan should with resistence and NOT have any wobble. If either replace. As for cap need to apply downward pressure while turning to get past safety lock.

Base of the upper radiator hose under what it connects to is the housing, that is where the thermostat is.