Set Appliance State:
Set Multimeter to AC Volts 
Isolate Mains Power to appliance as per TOPs P4
CH temperature selector to 0 position
DHW temperature selector to 0 position
CH temperature selector to 0 position
DHW temperature selector to 0 position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow appliance to self check and return to standby mode.
Measure voltage between the inline connector of the flame sense lead and the main earth point on the appliance.
Expected voltage approximately 27 volts AC.
Measure voltage between the inline connector of the flame sense lead and the main earth point on the appliance.
Expected voltage approximately 27 volts AC.
Isolate as per TOPs P4
and refit as necessary