Problem with Devi undertile heating in bathroom

Hope someone can help with ideas on what has 'gone wrong' with my bathroom Devi undertile heating. This was installed about 5 years ago, and has worked fine all that time. A couple of weeks ago, the floor was cold, but the Devireg 550 controller was still on and showing all the correct info - time, temp, element on etc. I checked all the connections, resistance etc. and could find no faults. Then a day or two later, the floor warmed up again okay. Now, the same thing has happened again. The system has a floor sensor and the wall thermostat/controller. Is it likely that one of these is becoming faulty?

Any help appreciated!

You'll need a multimeter or a proper voltage indicator (NOT a neon screwdriver)

Check the output of the controller when the floor is supposed to be on but is not heating. Do you get a voltage:
If no then you have a problem with the controller, the probe or the input supply
If yes then you may have a problem with the heating element. Turn off the power, remove the two cables that go to the heating element and measure the resistance in ohms of the heating mat.
If you get no reading then you may be in for a floor relaying. If you do geta reading then compare this with the value taken when the floor was laid.

Thanks for your response TTC. It seems that the floor heats up okay one day and not the next. I'll just have to catch it to test when it isn't heating, as you suggested.

The thermostat/controller would be by far the easiest to replace, so I hope it's that rather than the floor sensor. Certainly something is going wrong!!