La Crosse Technology WS-9125 - Manual (Page 10)

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4. The outdoor temperature from the first sensor (channel 1) should
then be displayed on the Temperature Station. Also, the signal
reception icon will be displayed. If this does not happen after 2
minutes, the batteries will need to be removed from both units and
reset from step 1.
5. Insert the batteries to the second sensor immediate after (within 10
seconds) inserting battery to the temperature station.
6. The outdoor temperature from the second sensor and the "channel
2" icon should then be displayed on the Temperature Station. If this
does not happen after 2 minutes, the batteries will need to be
removed from all the units and reset from step 1.