Check the sensor's own connection
For example, Google says the Nest Temperature Sensor supports only certain Nest thermostat models. It also notes that distance and interference can cause a sensor to disconnect even if it passed its first setup test. Moving the sensor closer and using the app's connection test are owner-level checks for those supported products. Nest sensor troubleshooting.
Your brand may use a different radio, app, sensor, or setup process. Use the exact model's instructions for battery type, battery replacement, pairing, and placement. Do not apply a Nest reset procedure to an unrelated sensor or perform a factory reset before saving the system settings and asking support whether that step is needed.
Separate three different problems
Record whether the sensor disappears from the app, the thermostat itself goes offline, or the furnace stops responding to a local heat request. Those are different starting points for support.
If only the sensor is unavailable, confirm through the owner's controls which temperature reading is currently being used. Ask how to select the thermostat's built-in reading or another supported sensor while troubleshooting. Do not assume the system automatically uses the room you expect.
If the thermostat also shows a power warning, document that separately. If the house is losing heat, make a service request about the heating loss as well as the wireless symptom. A successful sensor connection cannot establish furnace operation.
Confirm the room result
Once communication is restored, check that the intended sensor is selected at the intended times. Google's sensor setup guidance warns that using one room's reading can make other rooms uncomfortable. Nest sensor placement and control guidance.
Compare how the rooms actually feel before choosing a permanent sensor location. A remote sensor changes which temperature informs the control; it does not independently repair a room's airflow problem.
Your next step is to send the exact thermostat and sensor models, connection error, and results of supported owner checks to the manufacturer's support team. Include a separate heating-service request if local temperature control is failing.