Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Set temperature selector to MAX
Place multimeter probes on the gas valve connections
+ve probe to grey lead -ve probe to white lead
Mains reset switch On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

After approximately 8 seconds is voltage present across the connections of the gas valve?
Expected voltage approximately between 12 and 26 volts DC.
Note:
Voltage will only be present for 2 to 3 seconds if burner does not ignite.