Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Rotary Switch in DHW position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the DC voltage across the Violet wire and the Yellow wire at the gas valve solenoid.
Repeat the measurement using the Violet wire and the Brown wire at the solenoid.
Either measurement should be approximately 25 volts DC.
Repeat the measurement using the Violet wire and the Brown wire at the solenoid.
Either measurement should be approximately 25 volts DC.
Isolate as per TOPs P4
and refit as necessary