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
DHW potentiometer and the soldiered connection
(bottom left of the four connections located centrally).
Resistances measured will differ as temperature selector is turned.
Resistances should conform (approximately) with the settings below.
Minimum demand - 166 kOhms.
Medium demand - 217 kOhms.
Maximum demand - 262 kOhms.