Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Refit 4 wire fan cable to fan assembly
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset the boiler / cancel pump purge as necessary.
Allow the fan to run.
Measure the DC voltage between the blue wire at connector X4 and a suitable earth.
Expect approximately 4.5vdc.
Is the voltage measured as expected?
Note:
Test at PCB connector can be carried out on either the small holes or the larger silver soldered points.