1999 Isuzu Rodeo Question diagnostic port

When this vehicle went in for inspection the portable diagnostic tool worked and read the codes. When the mechanic hooked up the state computer to the car it read "the computer does not appear to be connected". The mechanic tried the diagnostic tool again and it worked, Then the state computer and nothing. The state computer then worked on another vehicle, he showed me everything and is a trusted known mechanic who admitted it didn't make any sense to him. I put a meter on the terminals and got only five volts from the red wire while the car was on. Should this be 12 volts? Any thoughts, the vehicle can't pass unless this connection can be succesful. HELLLP!