Set Appliance State:

Gas Off
Set Multimeter to Continuity Set meter to measure Continuity
Isolate Mains Power to appliance as per TOPs P4
Unplug M4 from PCB

Measurement:

With -ve lead connected to centre White/Brown wire, measure approximately 1.5 on white/purple wire and approximately 1.7 on White/Blue wire at disconnected plug..
With +ve lead connected to centre White/Brown wire, measure .OL on both White/Purple wire and White/Blue wire at disconnected plug.
Replace plug M4 after test.