it sounds like the thermostat sensor is not making a connection. See if it plugged in all the way
Are you getting heat out of the heater?

 

agree 100%. had this as well. the electrical connection on the harness side is known to deteriorate and break, or the wires can back out of the connector and not make a connection. I was able to jam them down and ensure contact was made, and made a note to replace the connector (p/n 12527542849) which I now have and will tackle soon.

When the connection isn't made the fan goes into fill-on blizzard mode and stays on for 5-10 minutes after you shut the car off. Safe mode, I guess.

 

 

Woke up today with battery completely dead. I bet the fan probably ran on/off all night. Confirmed the connection and fan still comes on randomly, even after a quick drive around the block. I also noticed it wasn't pulling the coolant in after a short drive that definitely got the engine warm. Any other ideas?

Overflow tank is filled to Max. Engine cold, When opening the vent screw on the thermostat, coolant trickles out.

Could it be a faulty thermostat?

 

I don't think a thermostat would cause that problem. It's hydro-mechanical and simply allows or prevents the passage of water down a certain path. if it didn't work, you would overheat, or take a long time to warm up. My money would be on the temp sensor or the wiring to it. But you said you checked the wiring. Are you 100% confident? Based on the work you did, that would still be my guess.

In other travels, I've run across people fixing odd fan behavior by swapping or replacing the fan relay. There may be 2? Not sure, but maybe worth tracking down. And of course, any codes?

 

Make sure you are getting heat out of the heater