Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Remove underfloor heating connector marked 'TA2' / FLOOR

Measurement:

Measure for resistance across the pins of the removed connector 'TA2 / FLOOR'
Expected resistance short circuit (zero resistance).