I’d put my money on your cab control module needing updated. If you set the temp on auto and turn it to 84 does it really crank the air out? Or does the blower fan run slow?
You need a shop with ESA software to run a HVAC diagnostic procedure. I am betting you will find active codes in CECU. I have a bad feeling that your problem is a $35 temperature door flap (actuator attaches to) that is going to cost $1200 because of labor. I did a post on it couple years ago if you use search function.
More likely a module issue Are the taps open on engine? I closed mine due to a leak to get home but was still getting some heat..
Sir I am having the same issue. Couldn’t solve it yet. I have read online that this could be a software issue. Please let me know how did you fixed yours ?
OK I know I am going to catch 4377 upside-down for this and please don't think I am calling you stupid because I am not Did you make sure the valves on the motor are all the way open. On my Pete I can shut off the heat to the sleeper and leave it on for the cab or vice versa or have them all open or closed. Just a thought......
I literally just had the same issue. Turned out my heater core was plugged up. I have a 2018 579 as well.
