Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Selector Switch in HOT WATER position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the Fan to operate and measure the voltage across PCB connection 66 on connector R (Blue wire, air pressure switch connection N/O) on the PCB to a suitable earth point.