Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect and remove the user control (display unit)
Remove the front cover

Measurement:

Measure the resistance across the L/H side of the CH potentiometer and the soldered connection (bottom right of the four connections located centrally).
Resistance will change with increased demand.
Minimum demand approximately 165 Ohms.
Medium demand approximately 220 Ohms.
Maximum demand approximately 270 Ohms.