Only arms when connected to USB

I am trying to troubleshoot a problem that occurred after a crash that bent a motor off my 3DR quad. Now the quad will arm only when I have the APM 2.5 connected to the computer via a USB cable. If the LiPo battery is connected to the quad and the USB cable is connected to the computer it will arm and the props spin as expected but if I disconnect the USB cable the props no longer spin. I am not able to arm the quad using only the LiPo battery (without the USB cable). Any ideas what the problem might be? 

Another, possibly related, problem is that I am not able to connect to mission planner using my 3DR radios (connecting with USB works as expected). I can load the radio parameters in the 3DR Radios configuration window in Mission Planner but when I try to connect I get a message “Getting Params … (sysid 1 compid 1)” and then a “Connect Failed” message with details “Timeout on read – getParamList 37 packets were received , but no parameter packets were received” followed by notes pointing to lines of code. The radios were working fine before the crash and they were not on the quad when it crashed so I expect it's a problem with the APM 2.5 and not the radios.

I'm at a loss as to how to troubleshoot these problems and any guidance would be appreciated.

Thanks in advance,


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  • I get the same similar problem except mine seems to be really strange. When I arm my pixhawk, it will have yellow blinking lights unless I move the USB connection to the USB port of the Pixhawk! I don't even need to connect it , all I have to do is move the micro USB from the computer to the micro USB port on the  PIxHawk's USB connector and it becomes all green and ready to go!

    Am i getting EMI interference etc etc?



    • I'm having this issue after previously replacing the Diode that had blown.

      Can only arm with the USB connected.

      Zener Anode       5.34v

      Zener Cathode    5.09v

      Voltage at Fuse   5.09v

      Any ideas?

  • I'm having this same problem.  Constant flash suggest it's ready to arm, but it won't.  It will only arm when connected to USB (whether Mission Planner is running or completely closed out).

    I checked voltage per the linked thread.  Not getting anything at A (top of diode), but getting ~5.3 at B (bottom of diode) and at C (fuse).

    Does that mean the diode is reversed (and bad), or that it isn't either of these components?


    This started suddenly, after multiple successful flights. 

    I'd been using the ESC's (all four) for my BEC.  I realize now that's not great.  The commentary runs from "only one is necessary" to "more that one is suicidal."  Is that a likely cause of this problem?

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