My 2016 Tahoe heat sometimes works and sometimes doesn't. seems to get luke warm when driving and takes awhile to heat up. sometimes it works great. I don't know if I'm chasing a fluid issue or electrical. My heater core doesn't seem to be leaking and my fluid is at a perfect level and the truck doesn't overheat.

 

if a no heat concern is coolant related
-check to see if a p0128 fault is set...thermostat fault
-what temp does the temp gauge read...normal is 80-100*C...there will be less heat until the coolant is at normal operating temp
-do you get more heat output at 2500rpm vs idle?....signs of air in the cooling system
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check the operation of the temp blend door actuator...if its broken a range fault will be set in the hvac module