I recently had a fault with mine. It displayed -40 on the unit but wouldn't blow any air at all. Made air hot and cold but only if I drove fast enough for it to be drawn in by vehicle speed.
After months of relay/fuse box/ outside temp queries it turned out to be a loose wire on the rear of the HEVAC control unit. Loose wire plugged back in, all works perfectly. No book on display, reads outside temp accurately and blows hot or cold air conditioned air...........If only all issues were that easy to fix!
Check behind here first.