Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Selector Switch to Hot Water and Heating
Remove the flame sense lead from terminal X501
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Using shrouded probes measure the voltage across
terminals at X501 and X500 on the PCB
Expect approx. 70 V AC.
Note:
This voltage should be present
regardless of which stage the boiler is in.