Pixhawk 2.1 issues

Am facing problems with my pixhawk 2.1
Issues include :
1.) after flashing the apm firmware into the flight controller it doesnt detect the flight controller giving an erro of trying to connect timeout in some seconds and i need the apm firmware to see if all other eternal sensors like optical flow sensor is properly working with the flight controller or not.
2.) after flashing the px4 fligt stack firmare and setting the SYS_MC_EST_GROUP parameter to lpe fusion i dont see any lpe parameter tree on the parameter list.

if anyone has found any alternatives for the first issue then please help me out. and for the second issue also.

For PX4 if you want to use LPE you need to flash an px4fmu-v3_default binary. If you grab the latest daily build of QGroundControl this should happen automatically. The px4fmu-v2_default binary is limited to 1MB and as a result doesn’t have LPE.

Hi, I am using pixhawk2.1,too. I am puzzling about whether px4fmu-v2 or px4fmu-v3 is right? what is the difference between them?

Thank you!