lennox rooftop unit

I,ve got a lennox rooftop heatpump unit with a kp17 comfort controller ,the controller is showing Pb com fault (communication error),the interconnecting wirings ok as i,ve connected the controller directly to the outdoor unit which leaves either the kp17 controller,kp18 interface or the kp01 master module at fault, has anyone come across this problem before if so what was the answer.
cheers
Redroge