Thanks for having this resource and to cklien7385 and others!
I am back on the road so far so good.
This same exact condition (everything mentioned in thread) occurred yesterday while driving my 2014 GMC Sierra SLE 5.3 L V8.
Left house 12/3/2024 to run an errand 15 miles from home - everything normal but had to stop for extended period for train crossing and noticed loud noise after a train passed - gauge temp 160 degrees and determined cooling fan was running at high speed, voltage 14.5 V on gage. Was 39 degrees in AM warmed up to 50 by the time I left. First real cold snap here in south MS.
Pulled over and googled and found this thread.
Hoses cold, coolant in expansion tank. Turned heat up to full blast and it was blowing hot air so figured I had coolant circulating and it would be safe to turn back and hit nearest parts store. Halfway toward home air went cold and check engine light went on. Pulled over and called for ride. No maple syrup smell indicating coolant leak and fluid in expansion tank. Fan running for 5 minutes after shut off. No visible leaks or odd smells but wasn't taking chances on vehicle with 175,000 miles.
Luckily I keep tools and spare parts in my truck. Had coolant and spare thermostat but wasn't sure if it was right one. Checked out you tube videos and additional threads on this board while waiting for ride and locating parts at Advance Auto.
Bought 2 gallons dexcool, new temp sensor and new 207 degree OEM thermostat and housing - $130 total. Returned to vehicle 30 mins later, released a little pressure on cap on white plastic expansion tank slowly with gloves and towel. Barely a hiss and no fluid overflow, coolant temp warm but not dangerously hot. Hose to thermostat housing from radiator felt limp and empty and cool as well.
Disconnected ground on battery and cleaned terminal and stub with sand paper and greased and reconnected (I was told this would reset check engine light at parts store), removed air lines (squeeze the pinch releases top and bottom simultaneously and pull back quick connects toward cab) and loosen 6" hose clamp on air handler, pull and peel back to access thermostat housing and water jacket temp sensor underneath the unit (driver side of engine). Disconnected 2" radiator hose clamp with channel locks, removed hose from front of housing and dumped warm coolant into plastic paint bucket I had - about a quart came out. Disconnected 3/8" clamp and hose on top thermostat housing and wire harness on sensor maybe 1" to left of thermostat housing. Removed and reinstalled temp sensor and aluminum crush gasket with 19mm box wrench. My sensor threads came with thread dope on them so I didn't tape it. Removed three 10mm bolts on thermostat housing, pulled off inspected and found thermostat stuck in open position. Cleaned up old rubber O ring and wiped the base, squeezed lower hose to force coolant and debris out. Reinstalled new unit and gasket - with just a 1/4 turn past snug as I noticed the base threads are in cast aluminum. Fortunately, everything I purchased matched the old and I only got a little grease and dirt on my hands, no burns or cuts or lost tools.
Added new dexcool to expansion tank (about a gallon) and started up truck. Engine light off and truck started warming up as normal in idle above 160 degrees and heat was hot fairly quickly. Headed for home with needle reaching just a hair under 210 degrees - normal operating temp.
Hope this help someone else! I am very much appreciative and for the moment relieved.
