Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove control box cover
Disconnect plug marked CH sensor
(see (1): 6 way connector with 3 red leads, 1 white lead and 1 black lead)
Remove control box cover
Disconnect plug marked CH sensor
(see (1): 6 way connector with 3 red leads, 1 white lead and 1 black lead)
Measurement:
Test 1:
With CH temperature selector set to maximum, measure the resistance across the red wire (connection 1) and the black wire (connection 2). Resistance measured should be approximately 9.5 kOhms.
Turn the CH temperature selector to minimum.
Resistance should reduce to less than 100 Ohms.
Test 2:
With CH temperature selector set at minimum, measure the resistance across the white wire (connection 3) and the black wire (connection 2). Resistance measured should be approximately 9.5 kOhms.
Turn the DHW temperature selector to maximum.
Resistance should reduce to less than 100 Ohms.
With CH temperature selector set to maximum, measure the resistance across the red wire (connection 1) and the black wire (connection 2). Resistance measured should be approximately 9.5 kOhms.
Turn the CH temperature selector to minimum.
Resistance should reduce to less than 100 Ohms.
Test 2:
With CH temperature selector set at minimum, measure the resistance across the white wire (connection 3) and the black wire (connection 2). Resistance measured should be approximately 9.5 kOhms.
Turn the DHW temperature selector to maximum.
Resistance should reduce to less than 100 Ohms.
Isolate as per TOPs P4
and refit as necessary