2017 Granite GU713 Engine fan stuck on and no A/C

You say ac compressor is good and checks out, but 0 heat difference, that tells us 1 of 2 things, either your ac clutch orcompressor arent actually working, or your orifice/expansion valve is stuck full open as if its just an open tube

Try hot wiring your ac compressor first

 

I’m not familiar with the system on that truck, but there are a few things I would check. First the High and Low Pressure Switches on the AC system. Either one of those failing can hold your fan on. In your fuse panel you should find a relay that turns on the AC with Defrost, might look there too. Had a problem with one of those in a KW, long storey, but it burned the truck to the frame rails in the end. The Emissions involvement is a mystery to me, but I would think that’s probably switched from the ECM. Sounds like you’re not getting a signal to the AC clutch for whatever reason. Those 2 switches, and the relay could all be the culprit there. Scary part in it is Emissions System can control fan function and somebody just had their hands in it. I’m assuming it was a NOX sensor you had changed, you should know if your box is up or not without a sensor. Couple things there you can look at anyway.