If I power up my Pixhawk with the external compass plugged into the I2C socket I get the Double flashing yellow: Failing pre-arm checks (system refuses to arm) indication. If I power up without the external compass connected I get green led and can arm.
Gerald,
It should display a reason on the ground station HUD (or OLED display if you’ve got one). that normally explains the reason and it could be one of a few things including “requires calibration” (if you haven’t calibrated it yet).
I have calibrated. I’ll try to calibrate again. Just a little more difficult with my very large hexa.
Not sure which manufacturer’s GPS/compass you’re using but I had a problem last weekend with a brand new mRobotics external compass that couldn’t be calibrated. I’ve replaced it and the new one is fine. I’ve also discussed with Jordi and they’re going to improve their factory checks.
I mention this only in case your compass is also from mRobotics.
Having similar issues with an mRo module @rmackay9. I can get it to calibrate and save offsets but when I reboot I get a prearm: compass not calibrated warning. Is that what you are/were seeing?
I do have another mRo unit to try.