Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to DC Volts 
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow fan to run and ignition sequence to begin, once the sparking has started, measure the voltage across the connections of the modulating solenoid.
Expected voltage is approximately 10 Volts DC.
Expected voltage is approximately 10 Volts DC.
Isolate as per TOPs P4
and refit as necessary