Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On

Measurement:

Is diverter valve flow adjuster open sufficiently?
Alter the manual flow adjuster control between fully open and fully closed and back to fully open.
Note:
Flow adjuster is situated on the inlet of the diverter valve.
Turn clockwise to close anticlockwise to open.