La Crosse Technology WS-9121 - Manual (Page 30)

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3. Using other electrical products such as headphones or speakers
operating on the same signal frequency (915MHz) may prevent
correct signal transmission and reception.
4. Neighbors using electrical devices operating on the 915MHz signal
frequency can also cause interference.
Note:
When the 915MHz signal is received correctly, do not re-open the battery
cover of either the transmitter or the Temperature Station, as the batteries
may spring free from the contacts and force a false reset. If this happens
accidentally, all units must be reset (see Setting up above) otherwise
transmission problems may occur.