Did you verify that the wires you connected go to the correct pins?
Blue/White or Black - Pin 13?
White - Pin 21?
It's very common to have the wiring incorrect. The correct wiring is to connect the TX of the EFIS-D10A (Pin 13) to the RX of the transponder. Many confuse this and connect "TX to TX" and "RX to RX". You should also connect the Ground of the EFIS-D10A (Pin 21) to a serial ground on the transponder.
Otherwise, this is known to work well between the two units. If it's not wiring or configuration (2 most common issues), then it could be in either the GTX-327 or the EFIS-D10A.
Just hooked up the D10A to my new Garmin 327 transponder. I selected format (4) on the Dynon, and have tried the Icarus on each of the input channels on the G327. I do not get a pressure altitude readout on the TXP. I have verified the wiring from the D10A to the TXP.
Any thoughts?
Thanks for the help,
Ron