The external AHRS is switched on but not set up

Stops itPosition and attitude estimate (EKF, AHRS)

What ArduPilot says:

ExternalAHRS: Invalid backend
MicroStrain5 ExternalAHRS no UART
ExternalAHRS: Incorrect number of GPS sensors configured for EAHRS
KEBNI: Serial Port Not Found!
VectorNav ExternalAHRS no UART
SBG setup failed

Also explained here: 6 more

no UART
VectorNav setup failed
SBG ExternalAHRS no UART
Unsupported ExternalAHRS type N
InertialLabs setup failed
InertialLabs ExternalAHRS no UART

An external AHRS is a separate navigation unit (VectorNav, MicroStrain, InertialLabs, SBG and others) that does the job of ArduPilot's own estimator, or feeds it IMU and GPS data. EAHRS_TYPE says one is fitted, but the driver could not start. “no UART” and “Serial Port Not Found”: no serial port is set to protocol 36 (AHRS). “Invalid backend” and “Unsupported type”: this firmware has no driver for that EAHRS_TYPE. “setup failed”: the unit did not accept its configuration. “Incorrect number of GPS sensors”: the unit supplies a GPS but no GPS in ArduPilot is set to take it.

What to do, most likely first

No external AHRS is fitted; the parameter came from another aircraft's file.
CheckEAHRS_TYPE is not 0, or AHRS_EKF_TYPE is 11.
FixSet EAHRS_TYPE to 0 and AHRS_EKF_TYPE to 3, then reboot.
The serial port the unit is wired to is not set for it.
CheckThe SERIALn_PROTOCOL of that port is not 36, or its baud rate differs from the unit's.
FixSet SERIALn_PROTOCOL to 36 and the port's baud rate to the unit's, and reboot.
The unit's GPS is not routed into ArduPilot.
CheckEAHRS_SENSORS has bit 0 (GPS) set and no GPS has its type set to 21.
FixSet GPS1_TYPE to 21 (ExternalAHRS), or clear bit 0 of EAHRS_SENSORS if you use a separate GPS. Reboot.

Parameters to look at

Parameter	What it is
`EAHRS_TYPE`	AHRS type
`EAHRS_SENSORS`	External AHRS sensors
`EAHRS_RATE`	AHRS data rate
`SERIAL2_PROTOCOL`	Telemetry 2 protocol selection
`SERIAL4_PROTOCOL`	Serial4 protocol selection
`GPS1_TYPE`	GPS type
`AHRS_EKF_TYPE`	Use NavEKF Kalman filter for attitude and position estimation

Where to do it:Board & Wiring Planner Starting Parameters

The code that sends it

ExternalAHRS: Invalid backend

libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp AP_ExternalAHRS::pre_arm_check() · Open in ArduPilot Copter-4.7.1

MicroStrain5 ExternalAHRS no UART

libraries/AP_ExternalAHRS/AP_ExternalAHRS_MicroStrain5.cpp AP_ExternalAHRS_MicroStrain5::AP_ExternalAHRS_MicroStrain5() · Open in ArduPilot Copter-4.7.1

ExternalAHRS: Incorrect number of GPS sensors configured for EAHRS

libraries/AP_ExternalAHRS/AP_ExternalAHRS.cpp AP_ExternalAHRS::pre_arm_check() · Open in ArduPilot Copter-4.7.1

KEBNI: Serial Port Not Found!

libraries/AP_ExternalAHRS/AP_ExternalAHRS_SensAItion.cpp AP_ExternalAHRS_SensAItion::AP_ExternalAHRS_SensAItion() · Open in ArduPilot Copter-4.7.1

And 8 more messages, sent from the same places.