Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
CH temperature selector to 0 position
DHW 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:
Reset lockout as necessary.
DHW temperature selector to maximum position.
Allow fan to run and burner to ignite.
Measure voltage across the connections on gas valve 3 (black leads, solenoid farthest to the left).
Expected voltage approximately 20 volts DC.
DHW temperature selector to maximum position.
Allow fan to run and burner to ignite.
Measure voltage across the connections on gas valve 3 (black leads, solenoid farthest to the left).
Expected voltage approximately 20 volts DC.
Isolate as per TOPs P4
and refit as necessary