Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow ignition sequence to commence and fan to run.
Measure the voltage at the orange wire (pin 5) to the purple wire (pin 9) at the fan PCB.
Expected voltage approximately 33V DC.