4+1 Quad-Plane lift motors will not run unless a usb lead is connected to my Pixhawk flight board

I have a very odd problem arming my Q plane in drone mode. With my computer connected via usb into the Pixhawk 2.4.8, I can arm the plane and select Q stab or Q hover. The plane arms and motors turn and continue after the usb is removed. Now, if I disconnect the usb lead and then apply main power to the system, the rear pusher motor works fine. Selecting Q stab, I hear the arming tone but the drone motors do not run. ever. In mission planner the drone is showing armed and I can see the servo menu motors 1-4 appear to spool up on the gui, but the motors do not turn.
I have my 4 main lift motors mapped to the aux op 1-4 and wonder if this may be the problem. But why is it necessary to have a 5 volt usb lead powered to make these lift motors run.

What are you using to power your FC?
A PM or a BEC ?
Maybe worth checking the voltage that is supplied to your FC.

Yea, I considered a low voltage so have tried both power module straight to pixhawk and also tried a bec powered to my receiver then on to the sbus input on pixhawk. My measured voltage on both is 5.1v

I think I know what’s going on:
The aux channels are usually not powered. So your signal going to the esc’s is missing ground (-) unless you connect the USB cable providing that via the USB port.

I guess you’ve got 2 options here-
a) re-map the motors to the regular output channels
b) run a negative wire from your BEC to the aux output or from the regular output across to the aux channel. ( But do not do this if you got servos attached to any of the aux channels)