Set Appliance State:
Gas Off
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to AC Volts 
Turn Hot Water Tap On
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Ensure gas is fully OFF, failure to do is a safety risk AND will cause misdiagnosis.
Measure the AC voltage between the fan terminals.
Full mains voltage should be present.
Measure the AC voltage between the fan terminals.
Full mains voltage should be present.
Isolate as per TOPs P4
and refit as necessary