Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Selector Switch in HOT WATER 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 Brown wire and Black wire on the Modureg. The voltage obtained should be approximately 24 V DC.
Allow the Boiler to reach its normal operating temperature and if necessary decrease the Flow of DHW, does the DC Voltage across the Solenoid modulate down as the DHW demands a reduction in Burner output.
Allow the Boiler to reach its normal operating temperature and if necessary decrease the Flow of DHW, does the DC Voltage across the Solenoid modulate down as the DHW demands a reduction in Burner output.
Isolate as per TOPs P4
and refit as necessary