Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Turn Hot Water Tap On
Selector Switch in DHW Summer position
Hot Water Thermostat set to Maximum
Disconnect flame sense lead from the PCB
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

With burner alight and before lockout occurs measure the DC voltage between Flame Sense Electrode (+ve) and one of the Modulation Coil terminals (-ve Grey). Voltage measured should be greater than 100mV DC.