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Thanks for giving me the opportunity to help! According to the manual, the batteries are the main cause for "no display". Are you saying you had another CM67 with no display, as well, that you tried here?
If you look at the first item on the troubleshooting chart "blank display", it lists a number of reasons for this, but all seem to relate to the batteries in some way.
The second thermostat works ok. I swapped it with the faulty one to check that power supply, heating cables etc were ok, and there was no problem.
I have read the trouble shooting chart
I have just checked the faulty thermostat again with new batteries and a meter - there is 3.3V supply to the circuit board - still no display. What do you suggest?
The CM67 thermostat is no longer available. Newer models use a floor sensor with different resistance - not compatible with CM67.
The resistance of the existing sensor is approx 128kohm at 19 degrees C
Can you suggest an alternative solution?