98.5 3500 24v. Start the truck today and defrost is on , fan at 3 out of 4. Fan speeds 1 2 and 3 operate at 3 and 4 is 4. Adjusting from floor to defrost wasnt working correctly and now just stays on defrost even in off position. My question is, could this be vacuum? Or is it more likely the controller itself. Thanks, Jim

Truck is stock at the moment.

 

Vacuum leak for sure for the HVAC system and why the mode is stuck on DEF. That's a safety measure to allow for DEF if the vacuum system should fail.

As for the fan speed that is a blower motor resistor. Just remove and replace. It install right next to the blower motor on the passenger side you can't miss it.

 

Thanks. Got'er back to normal.