The EKF stopped agreeing with its sensors (ERR EKFCHECK)

What ArduPilot says:

• ERR EKFCHECK (Subsys 16)

Code 2 (bad variance): for a full second the EKF, ArduPilot's position and attitude estimator, failed its check. Two of its compass, velocity and position test ratios were at or above FS_EKF_THRESH (0.8 by default), or velocity alone at twice that, or the position estimate was lost outright. The message is “EKF variance”, and the EKF failsafe (“ERR FAILSAFE_EKFINAV-1”) follows at once. Code 0: the check has passed again for a second. In flight, switch to AltHold or Stabilize and fly by hand: those modes do not use the position.

What to do, most likely first

1. The GPS went bad: jamming or spoofing, poor reception, or the fix was lost.

   CheckIn the log XKF4.SV (velocity) and XKF4.SP (position) rise towards 1 while GPS.NSats falls or GPA.SAcc grows; “GPS Glitch or Compass error” may come first.

   FixFly where the sky is open and the GPS is clear of transmitters; where jamming is expected, plan to fly without position.

2. The compass is disturbed by the motors' current or badly calibrated, so the heading is wrong.

   CheckXKF4.SM (compass) rises with throttle; the copter circles around its hold point before the error.

   FixMove the compass away from power wires, calibrate it outdoors, and run the motor-interference calibration.

3. Vibration is corrupting the accelerometers, so the EKF's own prediction is wrong.

   CheckVIBE.VibeX, VibeY or VibeZ above 30 m/s², or VIBE.Clip counting up.

   FixBalance the props, check the flight controller's mounting and remove whatever shakes or touches it.

Parameters to look at

Where to do it:Failsafe ExplainerSetup & Tuning GuideSetup & Tuning Guide