Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Activate maximum output mode (SC), CLICK HERE for guidance.
Allow burner ignite and establish.
Measure for DC voltage across the gas valve modulation coil.
Expected voltage approximately 18Vdc.
Note.
Voltage measured could be either negative or positive.