Originally Posted by DakotaTravler

Codes. There are several sensors in the system and items read. For example, amperage draw of the compressor is read. Pulls too much power, assumes there is an issue. Also the reservoir air tank pressure is monitored. If it builds too slowly, fault. And the list goes one. The basics can probably be figured out without codes though. First thing will be a hard-reset of the vehicle when you get it (disconnect battery for a few minutes, make sure you power down the car and WAIT a couple minutes before disconnecting! - Lets the computers power down to a safe state). Then go from there.

Originally Posted by DakotaTravler

Just disconnect, there is no need to connect them together. You can if you want, but science and physics says there is no point.