The flight mode change was refused

What ArduPilot says:

• Mode change to … failed: …
• ERR FLIGHT_MODE (Subsys 10)

The copter was asked to enter a mode (by the switch, the ground station, a mission or a failsafe) and refused; it stays in the mode it was in and the buzzer gives a low tone. The first blank is the mode, the second the reason, which has its own explanation. In the log it is “ERR FLIGHT_MODE” with the number of the refused mode as the code (3 Auto, 5 Loiter, 6 RTL, 16 PosHold, 27 AUTO RTL). While disarmed nearly every mode is accepted, so a switch position that worked on the bench can be refused once armed.

What to do, most likely first

1. The reason after the colon says what is missing. Most often it is “requires position”: no position estimate good enough for Loiter, PosHold, RTL or Auto.

   FixWait for a position before switching to a mode that needs one, and keep a mode that needs none (AltHold or Stabilize) on the switch to fall back on.

2. The mode number is not in this firmware (“No such mode” comes with it): the feature was left out of the build for this board, or the number comes from another vehicle type.

   CheckCompare FLTMODE1 to FLTMODE6 with the list of modes; the Firmware Inspector shows which modes a firmware file was built with.

   FixChoose a mode this firmware has, or build a firmware that includes the one you want.

In Plane this error is written only for a mode number that does not exist; a refused change there says “Flight mode change failed”.

Parameters to look at

Where to do it:Setup & Tuning GuideFirmware Inspector