Carrier Transicold Maxima 2
Error message: hi press


While helping out to a friend fixing a trailer, I was misled by the error message displayed on the control box.
I have guided my friend to check all the pressure switches and wiring on the discharge side of the system with no luck.
I felt a bit silly when the problem came out to be refrigerant shortage.
As soon as the system was charged with refrigerant all started running perfectly.
I could not obtain a service manual for this trailer or a circuit diagram, therefore I could not find out why the system was displaying hi press message while it was completely short of refrigerant.
Any comments?

Guner