Why does my Komatsu D37EX-21A have no travel and a communication error?

[Robert M.]

My Komatsu D37EX-21A starts and the blade works, but the dozer has no travel at all. It shows a DAFRKR E03 communication error, while the HST controller, instrument cluster, wiring harness, powers, grounds, and 120-ohm resistors have been checked or replaced. What else should I check for this Komatsu D37EX-21A no-tram problem?