Tiltrotor plane tricopter - no tail motor or servo working for yaw?

I am setting up a bench test of tilt tricopter plane.
Motor 1 and 2 working and tilt servo responds to Q stabilise but no response for motor 4 as tail motor and no response of the tail yaw servo. (No response in the servo output menu).
Should the yaw servo be controlled by the rudder tx output in manual mode?

Arduplane 3.9.3 on pixhawk

settings

Q_ENABLE 1
Q_FRAME_TYPE 1
Q_FRAME_CLASS 7 (“Tri-Motor Tilt Plane”)

Q_TILT_MASK 3
Q_TILT_TYPE 0
Q_ASSIST_SPEED 0
Q_TRANSITION_MS 0

SERVO1_FUNCTION 4 (Aileron)
SERVO2_FUNCTION 19 (Elevator)
SERVO3_FUNCTION 0 (disabled) or 70 (Throttle)
SERVO4_FUNCTION 21 (Rudder)
SERVO5_FUNCTION 33 (Motor 1 = ESC right motor)
SERVO6_FUNCTION 34 (Motor 2 = ESC left motor)
SERVO7_FUNCTION 0
SERVO8_FUNCTION 36 (Motor 4 = ESC rear motor) -not working?
SERVO9_FUNCTION 0
SERVO10_FUNCTION 0
SERVO11_FUNCTION 39 (Motor 7 = tail servo for yaw)
SERVO12_FUNCTION 41 (Motortilt = Front motor tilt servos) - not working?
SERVO13_FUNCTION 0
SERVO14_FUNCTION 0

thanks
M