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I have a Honeywell T8131C programmable thermostat. Our power went out yesterday, and now the "Bat Lo" won't go away. We replaced the batteries, but the "Bat Lo" indicator stayed on even after changing to new batteries. As per the troubleshooting guide, we took them out, waited an hour, and then reinstalled new batteries. Still, the "Bat Lo" indicator is still on. How do I get it to go away because I can't adjust the thermostat with it that way?

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  • Anonymous Mar 29, 2009

    I tried sticking the batterries in reverse polarity, but it did not work.

  • cz8729 Apr 12, 2009

    I have the same problem and tried the reverse battery trik, it did not work.

    There is catch though, when I switched the palarity and put the batteries in, the power was always on, even I put the second switch on the right on off position. Did I do any thing wrong?

    I had never had the unit cycling through any checking. Maybe there is a problem right there. How to turn it off completely? The thing is installed on the wall and even you un-plug the battery, it operates on the AC power of its own. Maybe I should shut down the power of whole house, huh?



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I tried all the tricks mentioned previously to no avail.

These are the steps I performed to get it to work:

1. Remove batteries and make sure the battery contacts inside the unit are clean.

2. Disconnect all the wires from the back of the unit. Be careful not let any of the bare wire ends to come in contact with each other. Make sure you mark the wires or verify that the color code matches the markings on the rear cover:
W=White wire;
Y=Yellow wire;
G=Green wire;
R=Red wire (low voltage DC); and
C=Common (wire color varies)

3. Leave the unit disconnected with batteries removed for about an hour to make sure all the memory drains. I waited an hour, but it may not require that much time.

4. Install the batteries in the correct positions per the instructions. The unit should perform a power up display test, then the display will flash the time and temperature.

5. Set the clock and the display will stop flashing.

6. Slide the switch to cool and/or heat and program the desired weekday/weekend settings.

7. Slide the switch back to off once all weekday/weekend settings are made.

8. Reconnect the wires that were removed in step 2.

9. System should be operational at this time.

Posted on Apr 22, 2009

  • TexFix Nov 30, 2010

    This took about 30 minutes for the memory to clear. This process also works if you re-wire the unit back in first, then re-insert the batteries in their normal orientation. After that, you can then set the clock and the display will stop flashing. Finally, you can re-program all the settings you may need. Enjoy....

  • LuckySuperMommy
    LuckySuperMommy Mar 26, 2014

    I had the same problem after a power outage. I essentially did the same thing to fix it, but instead of re-wiring, I just cut the power to the thermostat by removing the batteries and turning off the power at the breaker box to the unit. I waited about 30 minutes then put the batteries back in, set the day and time, and then restored power. That fixed the problem.


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I spoke to Honeywell...the fix is really quite simple! They asked me to clean the metal battery posts (in case there was corrsion). That didn't fix my problem b/c I had cleaned the corrosion previously. Next, the guy had me insert new batteries in REVERSE polarity and leave the unit in the off position for 10 seconds. He then had me take the batteries out and insert them in the correct polarity.

The unit cycled through its checks and worked perfectly!

Easy fix. Hope this helps someone else!


Posted on Dec 10, 2008

  • tescuadra77
    tescuadra77 Jul 06, 2012

    Thanks for the advice! I had the same problem with my Honeywell thermostat! Thank God I did not need to go through all the trouble of rewiring it. All I need to do was to clean the corroded battery posts then put in new batteries and voila! It's working good now!


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I tryed the timdmurphy advice of unhooking the cables and I waited 20 mins and it was gone.
Do not buy a new thermostat before trying this one

Posted on Sep 20, 2009

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Replace the thermostat.

Posted on May 29, 2008

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I have a Honeywell T8131C thermostat and I set it to come on at 6;00 in the morning to come on but it does not come on what is my problem

Posted on Jan 10, 2013

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