The Lux TX500U thermostat seems to have a bug where it reports low battery even after batteries were replaced – the top right of the screen will read “Lo Batt”. To fix this issue follow the below steps.

Taken from an amazon review:

Perform if installation of known fresh batteries does not resolve the flashing Lo Bat condition. You might want to read through the procedure before starting. To perform the Hardware Reset, proceed as follows:

1. Remove thermostat from its wall unit as if you were changing batteries and install fresh batteries. (I have not tried this with rechargeables so, ensure the batteries are of the 1.5 volt alkaline variety or equivalent as newer 1.5 volt battery chemistries might be available. You might try with rechargeables, but might have to perform the procedure at more frequent intervals throughout any given season.)

2. Make sure the switch positions on the thermostat are in the following positions:
–Temp Slider: OFF
–Set Slider: Air Filter
–FAN slider: can be in AUTO or ON, doesn’t really matter

3. Hold the thermostat in each hand so that the UP/DOWN buttons on the front can be depressed with each thumb. No need to press just yet.

4. While holding the thermostat in this manner, rotate it slightly toward you to locate the HW RST button on the back in about the center of the circuit board. You are now ready to RESET!!

5. Simultaneously PRESS & HOLD the UP/DOWN buttons on the front and while continuing to hold, then press and release the HW RST button on the circuit board. Continue holding the UP/DOWN buttons on the FRONT until 00 appears on the display.

6. When 00 appears, you may RELEASE the UP/DOWN buttons, then press & release the HW RST one more time.

7. If performed correctly, Days, Filter and 90 should now appear on the display. If you do not get this result, repeat 1 through 6 ensuring slider switches are in the correct positions and the reset sequence is correctly followed.

8. If #7 is successful, slide the Set Slider to RUN. The display should now show current ambient temperature, flashing display for programming time and temperature settings and the maximum number of bars for a full battery. If using rechargeables such as NiMH rechargeables, you might observe fewer bars for a full battery.

9. Hardware reset is complete and you can now reattach to wall unit and proceed with programming IAW the user guide.