Rough downshift linked to HVAC ?

[Simon101019]

Hey guys

 

My work truck is a 2017 Sierra 1500 with the 5.3

 

The truck has a weird condition. When the cabin fan is on, no matter which position the air is blowing or fan speed, downshifts are extremely rough. 
 

When the fan is turned down to zero, downshifts are fine. 
 

Turn the fan to any position other than zero... Rough downshifts again. 
 

I cannot compute why this is happenning... but I have tested it so many times, I am certain the downshift issue is 100% related to having the fan on. 
 

Any wild guesses as to what this could be ?

[rlcole321]

'No Pennies!", disconnect the battery overnight.

[Simon101019]

I will try that, a good reset wouldn't hurt !