Safety critical RF remote control protocol?

Coupled remote control with main unit. Both ends are transmit and receive using Nordic semiconductor nRF905 tx/rx chip sets. The synchro routine seems to work , requiring 2 way link both ways, to synchronous codings between both units. But whatever button is pressed , opens up the serial data line on the main unit , but non varying with whatever button is pushed. Uses Atmel ATMEGA68 uC on the hand unit and 2 x ATMEGA168 on the main unit . One probably for synchro and decoding functions and the other for opening out/monitoring to/from the controlled operations. Otherwise uses 4066 and ULN2003A to buffer out to relays and powerfets for pulsing/speed control. So pinouts available for all ICs involved Is the failure likely in the decoding / fanning-out operation or some feedback monitoring signal not getting to the main unit Tx as confirmation/handshaking? How to narrow down to which, no schematic available, and thick conformal coating over SM does not help