Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect the DHW Flow Switch PCB connector PL4
Boiler On/Off Switch ON
CH Mode Switch in its 24hrs position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Has the diverter valve now operated with the DHW flow switch disconnected?
Note:
Honeywell:
No or little resistance should be felt when the manual operation lever on the Diverter Valve is moved to its open position.
Sunvic:
Remove actuator head, flat of 'D' should lie across the line of the flex.