I (and others) have a problem where initial powering of the Pixhawk (legacy) does not post all the terms need to ARM the Pixhawk. The only workaround is to cycle power, then the Pixhawk properly posts all pre-ARM parameters and allows for ARMing the Pixhawk.
My Quad after a Power Cycle, the following posts in MP posts:
2/8/2021 3:59:20 PM : PreArm: Hardware safety switch
2/8/2021 3:59:12 PM : EKF2 IMU0 is using GPS
2/8/2021 3:59:12 PM : EKF2 IMU1 is using GPS
2/8/2021 3:58:52 PM : u-blox 1 HW: 00080000 SW: 2.01 (75331)
2/8/2021 3:58:51 PM : PreArm: Hardware safety switch
2/8/2021 3:58:50 PM : GPS: u-blox 1 saving config
2/8/2021 3:58:47 PM : EKF2 IMU1 origin set
2/8/2021 3:58:47 PM : EKF2 IMU0 origin set
2/8/2021 3:58:38 PM : EKF2 IMU1 tilt alignment complete
2/8/2021 3:58:38 PM : EKF2 IMU0 tilt alignment complete
2/8/2021 3:58:37 PM : EKF2 IMU1 initial yaw alignment complete
2/8/2021 3:58:37 PM : EKF2 IMU0 initial yaw alignment complete
2/8/2021 3:58:35 PM : GPS 1: detected as u-blox at 115200 baud
2/8/2021 3:58:34 PM : Barometer 1 calibration complete
2/8/2021 3:58:33 PM : Calibrating barometer
My Quad on initial power up the following posts in MP:
2/8/2021 3:39:12 PM : PreArm: EKF2 Yaw inconsistent by 24 deg
2/8/2021 3:39:12 PM : PreArm: Hardware safety switch
2/8/2021 3:39:09 PM : Frame: QUAD
2/8/2021 3:39:09 PM : RCOut: PWM:1-12
2/8/2021 3:39:09 PM : ChibiOS: d4fce84e
2/8/2021 3:39:09 PM : fmuv2 00330017 31345103 36313935
2/8/2021 3:39:09 PM : ArduCopter V4.0.6 (13b6478d)
2/8/2021 3:38:44 PM : Frame: QUAD
2/8/2021 3:38:44 PM : ArduCopter V4.0.6 (13b6478d)
2/8/2021 3:38:42 PM : PreArm: Hardware safety switch
2/8/2021 3:38:39 PM : Frame: QUAD
As I understand, this is a temp. issue. Wouldn’t this warrant the development of a PR?