Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Purge Switch in NORMAL position
HW Switch in ON position
CH Switch in OFF position
HW Potentiometer set to maximum
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Ensure that the Secondary Heat Exchanger is cool enough to demand a full burner output.
Allow the Fan to operate and the Burner to ignite.
Measure the Voltage across the two Violet wires on the Gas Valve Modureg Solenoid.