| What is the outside temperature?
Many batteries loose power at low temperatures, it is mostly notable below freezing, but even above the difference uses to be quite significant.
So the batteries may be still good, but just their performance deteriorates at cold.
Other possible cause could be a humidity: I assume the signal has to travel through some walls. Normally, when everything is dry, the signal may be just sufficient, so it works. Then when some material sucks in some humidity, it causes RF attenuation and the signal then drops below the minimum level necessary for successful data transmission. I would guess, when you will hook up the oscilloscope directly on the receiver demodulator comparator, you will see quite a lot of disturbance even when it "works".
The thing is, the sets usually do not feature any means of signal quality indication, so you do not know, how much RF margin you have. Normally they work seemingly fine even when 4/5 of the packets are getting lost. The problem is, when this figure becomes worse, they loose synchronization and that means communication loss.To save receiver battery, the receiver module is ON, so able to receive anything only brief time around the expected data transmission packet, all that design assumes the next packet will be transmitted after exactly defined time, the timers are then synchronized with each transmission. The problem is, when you get transmission errors (due to weak signal,...), the resynchronization does not happen, so with even slight mismatch between the receiver vs transmitter mismatch the receiver turn ON window misses the transmission packet and that means the communication gets lost.
The problem may get worse with extreme temperatures with yet another reason: The crystal frequencies tend to drift a bit with temperature, both the carrier frequency, as the main sequence controller (timing the transmission windows). At normal temperatures there is quite perfect match, so the reciever is perfectly tuned, as well as the synchronization get lost after way longer time after last resynchronization, so allows higher fraction of lost packets without loosing the sync. At extreme temperatures the carrier shifts, so the receiver becomes less sensitive, plus in case of packet loss the synchronization get lost way quicker. Both leads to more likely loss of connection from the sensor to the indoor unit.
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