2012 Duramax no heat

[esfd243]

A few weeks ago I had no heat on the highway. Drove over 300 miles and Luke warm. Engine temp gauge looked normal. Diagnostics scan showed a code that dealer told to change the thermostat. After finding 2 thermostats I changed both, flushed the whole system that was only 3 years old with 70k miles.
Now engine temp gauge looks higher. We have some heat for short distances. Get on the highway and it goes cold. Turn up the temp, immediate heat comes out. But not for long. Turn down the temp it goes cold. I don’t get it? What do I look at next?

[Skidus1]

Had to change out the hvac actuators in my wife’s Impala. Something to look for if the engine temp is good.

[Grit_dog]

Sounds more like something in the hvac controls than than the engine getting up to heat. If you have hot coolant flowing in and out of the heater core (easy to check) then it's a problem in the cab likely, not the engine.

[Bigfoot_affair]

You didn't mention if you have the manual or auto temp controls? If auto it could be the aspirator is failed.
What codes did you have?

[bob213]

If you don't get your answer here try this:
Duramax forum