Block Heater issue
In any event, they do not have enough BTUs to overheat anything. The core plug element is about 250 watts and the inline pump unit is about 700 watts. Way less than 1hp of heat. Most likely the programmers at GM were asleep and didn't think about block heaters and the ECU is freaking out
Assure that the coolant temp sensor has the correct resistance/per temperature. Such chart is easily available with a search. I find GROK helpful in such matters. You can easily replace the sensor with a known good USED sensor from a junk engine.
I have 5 GMT800 in my daly fleet rotation, all are 20+ years old, using original sensors, no failures noted except mechanical ( physical breakage) during a header installation.. I habitually use engine block heater for 2 or 3 hours before morning starts for half the year ( Alaska winters) and have NEVER seen the symptom you have described.
ALL of which is the long way of saying : CHECK YOUR WIRING and Connections.






