Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to mVolts DC Set meter to measure DC mVolts
DHW Demand
Disconnect flame sense lead cap from the sense electrode and leave in a safe position
Remove control box cover if necessary
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage between the end of the flame sense electrode and the neutral at the incoming mains terminal strip.
Expected voltage approximately 250 mV DC or greater.
Note:
Voltage will only be present for a very short time while the burner is alight so it is advisable to monitor the voltage over several ignition attempts. If the boiler locks out it will be necessary to reset it to continue testing.