05 5.4 239k
Replaced the water pump (Auto Zone),thermostat and flushed coolant.
Been through several heat cycles to get the bubbles out. 75 miles or so temp gauge pegs and I get a p1299 code and truck goes into limp mode. Key cycle returns gauge to normal and eleminates limp mode
External non contact temp reading ( lazer temp meter) show 210° at CHT sensor. The other bank reads 198.
Tstat housing and coolant crossover pipe are reading 195 so I'm sure the new tstat isn't defective.
Not sure what the cause is. I'd guess CHT sensor was bad if I didn't have dissimilar temps

 

Further research has me thinking the flush dislodged what was plugging a leaking head gasket. The coolant got nasty fast, a matter of months. Combustion gasses contaminated the coolant that caused it to become dirty enough to stop the leak untill I flushed it out.