Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Boiler Switch in ON position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage across Brown and Blue wires on Fan Connector Block.
Note:
Measurement should be taken on output side of connector.