Kit Jackson wrote

I've got a Fisher and Paykel fridge freezer model E240B but the
thermostat isn't working. The motor runs continuously and eventually
even the fridge compartment freezes up. I've taken the thermostat to
pieces as much as I can. The adjustment knob connects to a 1K
potentiometer that is working fine. Tucked behind the cooling plate are 3
sensors of some sort. One black one about 20 mm long. Is this the
thermocouple?

More likely to be an electronic temperature sensor.

The other 2 are discs about 10 mm diameter. I thought
at the start that they might be magnetic designed to stick
to the back of the cooling plate but they aren't magnetic.

They look like just where the wire goes thru the wall.

Has anybody any idea what might be wrong.

Either that temperature sensor has failed or what
is reading it has failed. Or a wire has come adrift
where you can't see it.

How can I test these components?

By replacing it is the obvious way.

What are they?

Photos he-
https://ibb.co/dcCyqPt
https://ibb.co/pLwnRHm