Set Appliance State:
Mains Power Off
Gas On; Pilot On
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Boiler Operating Switch in HW position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With the DHW tap fully open measure the DC voltage across the modureg.
The voltage obtained should be approximately 20 volts DC.
Decrease the flow at the DHW tap to assist temperature rise.
Does the DC voltage measured modulate down as the DHW temperature demands a reduction in burner output?
The voltage obtained should be approximately 20 volts DC.
Decrease the flow at the DHW tap to assist temperature rise.
Does the DC voltage measured modulate down as the DHW temperature demands a reduction in burner output?
Isolate as per TOPs P4
and refit as necessary