Strange that it would go out while driving. One moment I'm driving at 50MPH with the coolant temp reading 168F, then a moment later, what's in the picture I posted. Also on way back to work, after about 1 mile, it came back on. Will drain coolant, swap both thermostat & sensor at same time, & add new coolant being sure to burp the baby

 

The thermostat opens at 180F should close about 185F to 189F

the temperature sender is rarely at fault. But if you have a new one , put it in!

Burping is easy, if your driveway sloops just park the front bumper to the high ground, if you have flat level parking stop, simply jack up the passenger front side. Park it over night and check the coolant cold in the morning, there shouldn’t be any air under the radiator fill cap.

 

Ugh..... absolutely zero dexcool in the system, just pure water

 

Drain it out, use the radiator drain and the water pump drain plug.

 

Only drained about a gallon total, no tint at all, just crystal clear water. Replaced the gallon with Prestone Max concentrate - though I grabbed the wrong bottle. I grabbed the "All Vehicle" (bright yellow/green) version instead of the orange Prestone DexCool version. It's supposed to be compatible with & even mix with DexCool... From the Prestone website:
Prestone MAX will work in your vehicle:

• It’s guaranteed to work in All Vehicle Makes, Models & Years with Gas or Diesel engines.
• It mixes and works with all antifreeze + coolant colors and formulations including OAT, POAT, HOAT & IAT; G05®, G11®, G12®, G12++®, G13®, G30®, G40®, & GG40®.

It won't be in for long, once the GM thermostat gets here - looking like the end of next week - I'll swap out the Murray & refill again, though this time, I'll do more of a flush & get the DexCool version. Used the entire bottle, then filled bottle with purified & filtered water with 0.0ppm total dissolved solids. Instead of the burp method posted, I just started it up & let it heat up to 185F with the heater on max. I then refilled with water to top, then drove for 50 miles, stopping about every 5 miles to top up (did not need refill for last 15 miles) & swap 3rd brake light bulbs for testing purposes.

As soon as I put the Murray thermostat in, the trouble code automatically went away, didn't even have to clear it, though I did have to drive about 10 miles before cruise control started working.

Was NOT able to reach the ECT Sensor. I can see it, but unable to get to the connector to remove it, just a really awkward position for me to reach. Will keep an eye on things to see if ECT shows any signs of problems.