I have worked with these 433MHz wireless temperature transmitters, various brands.
La Crosse are absolute garbage for accuracy (bad firmware),
Oregon Scientific are the best so far but hard to find since they were bought out. I think only a few contract manufacturers are making these.
They are not using an A/D converter, instead it's an RC circuit and measuring the ramp times using a reference resistor and cap, compared to the thermistor. Those reference parts might be out of spec - instead of blaming the thermistor. Or, I found out my neighbor has a transmitter in a bad location and I was displaying that instead of mine. I used
SDR with
RTL433 to monitor the local devices to figure it out.
You can remove the SMT thermistor, connect a known resistor or potentiometer in place and see what values are transmitted, to reverse engineer the thermistor specs that are used.
Example with the crappy La Crosse transmitters, anything less than 11kΩ gave +60°C, 50kΩ +22.0°C, 100kΩ +9.2°C, 221kΩ -7.00°C, 500kΩ -22.3°C which all works out to a common 50kΩ/25°C thermistor beta 3950. Two La Crosse transmitters never gave better than +/-8°C accuracy at low temps so I tossed them in the garbage.
For the Oregon Scientific/Radio Shack I just added a series trimpot and calibrated it, so far it's within 0.1°C and works very good. I think it uses a 10kΩ/25°C thermistor.