Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Gain access to PCB (use insulation mat as necessary)
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltages across Black and Blue wires of the JP1 plug on the PCB.
While the fan is running expect approximately 5.5V DC.
Note:
Fan may only run for approx 5 seconds, use
ON/OFF switch to repeat test as necessary.