The 4202-DR or 4202-DS flashes S6 ERROR (5G, SG or 56) and then the display goes blank.
This is a normal part of the boot up sequence when there is no flow computer C program installed. This will happen after a cold boot or if the RAM battery has failed and it has lost it's C program.
You will need to connect to the 4202 with RealFLO, install a flow computer and a configuration that has elements setup for the Display Module. The Display Module depends on how the flow computer is configured so if nothing is configured, it won't display anything.