Im driving an Buick Lacrosse 2005. 3.8L V6
The other day i was driving and my Coolant temp gauge went flat to the bottom on the dashboard. So my best guess well maybe it was the thermostat that went out.. i replaced the termostat and voila it read the temperature of my coolant again. Problem fixed. So then a few days later my coolant temp dropped flat again on the dashboard. Now im thinking its the sensor.. any thoughts? Thanks

Series III motor btw

And it blows hot air aswell

Sounds like a bad connector, wire, or coolant sensor. When you disturbed it while replacing the thermostat you temporarily "fixed" it, then a few shakes and rattles later un-fixed it again.

If I were you, I'd find the problem first before blindly replacing stuff.

I would check the wiring to the sensor it may have a bad spot in it. Could be a bad connection either dirty or a break in the wire inside the insulation it may be the sensor or the gauge. Try, with switch on engine off, unplug the sensor and then try touching the wire to ground and have someone watch the gauge if it moves probably sensor but check that wire it may have a hidden broken spot in it. Try to wiggle the wire before you unplug the sensor with engine warm having someone watch the gauge. Hope that helps

This is also straight from the era of bad stepper motors in gauges. If you can display coolant temp from your dash, do so. If it'* where it should be, and the gauge is flipping out, it'* the motor in the cluster. If the coolant temp readings are jumping around with it, the answer lies elsewhere. If you cant display coolant temp, you can read the live data with a scanner too and it should tell you something