Set Appliance State:

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

Measurement:

Measure the voltage between each of the wires, in turn at the terminal block at the bottom of the boiler to pin 9 at the fan board.
Expected voltage approximately 35V DC.
Is the voltage present on neither, one, or both wires?