Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Water Off
Boiler drained and discharged

Observation:

Remove and inspect all the control lines between pump, servo valve and diverter valve.
Ensure control lines are in good condition and that there are no blockages in either the control lines themselves or where they enter the servo valve, diverter valve and pump bodies.
Are the control lines and valve body entry points clear of restrictions and blockages?