Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect red leads from sensor

Measurement:

Check resistance across sensor spade terminals.
Resistance measured should be less than 21kOhms.
Note:
Resistance measured will vary according to boiler temperature, the colder the boiler the higher the resistance.
If the boiler temperature is above 9 degrees C then frost protection should not be activated.