Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Turn/set the Hot Water Control (potentiometer) to approximately mid position

Measurement:

Unplug the connector PL 10 from the modulation control board and test the resistance across the black and grey wires (test at the plug).
The resistance should lie between 1200 to 2100 ohms.