In addition to time and date readouts, indoor-outdoor thermometers provide a home or property owner with temperature readings inside the home as well as ambient readings from outside. There are several make and models of the units that operate by hanging a remote sensor outdoors to gain accurate readings. At times, issues may arise that affect a correct outdoor reading, provided the unit is showing any readout at all.
The simplest approach to figuring out why the unit won't read outdoor temperatures is to consider swapping out the batteries in the sensor for new batteries. As battery power fades, the sensor has trouble relaying info to the unit since the data has to "travel" through doors and windows. A change of batteries gives the unit fresh juice. Also of note is the zone you live in. Extreme cold or below zero temps over a duration has a draining effect on batteries, so when you change them out, use two AA lithium batteries, which are resistant to drastic cold.
If you swap out the batteries in the sensor, don't forget to reset the unit by pressing the "Channel" button on the rear face of the display unit. You need to hold it down for a full count of three to reset the unit. You'll need to press the "TX" button for the display unit to search for and find the sensor's signal. You also need to press the "Mode" button three times; press it once and it gives you the humidity and temp reading, twice reads only the humidity and three displays the temperature only.
The placement of both the sensor and the display unit play a large part in accurate outdoor readings. The display unit needs to be placed ideally near a window to allow for easier reception. Concrete or metal paneling have a tendency to disrupt the flow of the signal, so you want to place the unit and the sensor in such a way that the signal path encounters the least amount of resistance. Try placing the sensor up high on an outside wall, away from any electrical interference from satellite receivers and the like. If need be, keep trying a few different locations until you find the right spot that picks up the signal.
Check the Sensor
If fresh batteries don't do the trick, the sensor may have become damaged. While the sensors are manufactured to be resistant to high heat and precipitation, realize they aren't weatherproof and that rainwater may have seeped into the unit and corroded some internal components. If you suspect this may be the case, you'll need to purchase a replacement unit from the manufacturer and follow the instructions to configure the new sensor to the wall display.