Set Appliance State:
Mains Power Off
Mode selector switch to Off/Reset position
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Hot water temperature control set to maximum
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Mode selector switch to DHW position.
Allow fan to run and burner to ignite.
Measure the voltage across the modulating solenoid terminals.
Voltage should be greater than 10 volts DC.
Allow fan to run and burner to ignite.
Measure the voltage across the modulating solenoid terminals.
Voltage should be greater than 10 volts DC.
Isolate as per TOPs P4
and refit as necessary