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AC Not Working After Thermostat Battery Change: Reasons & Solution 

It is not so uncommon that your HVAC appliances are not responding after you’ve changed your thermostat’s batteries. In most cases, you can troubleshoot and rectify the problem easily.

Your AC is most probably not working after you’ve changed the thermostat’s batteries because you’ve inserted the wrong batteries or have placed them incorrectly into the battery holder. The AC is not working as a result of a malfunctioning thermostat.

To make it easier for you to troubleshoot, we’ll discuss most of the common reasons why the AC doesn’t work and how to rectify these problems. Remember that if your AC is not working after a thermostat battery change it generally means there is a problem with the thermostat and not with the AC as such. 

The troubleshooting centers on steps to get the thermostat working again. If the thermostat is responding and the connections between the thermostat and the HVAC appliances are in place, your AC should start working again. 

Table of Contents

Reasons why your AC is not working after a thermostat battery change:

  • The thermostat has not been reset after changing the batteries
  • Wrong batteries have been used
  • Batteries have been inserted incorrectly
  • An acid leak into the thermostat
  • The wall-mounted thermostat is not making contact
  • The thermostat might have been accidentally set to “heat”

The thermostat has not been reset after changing the batteries

In many new electronic thermostats, a small battery on the thermostat is used to hold the thermostat’s settings in memory if the power fails or the batteries are changed. If this small battery is functioning there should not be a reset problem after the main batteries have been changed. 

Unfortunately, this little battery is rarely checked and over time it might stop holding the memory. Thus, even relative new HVAC thermostats sometimes have to be reset manually after new batteries have been inserted. This should be the first thing to do when the AC is not working after a battery change.  

Generally, it is a simple procedure to reset a non-programmable digital thermostat.  Just take off the face of the thermostat and press the reset button for five seconds. If you can’t find the reset button, take out the batteries again, wait five seconds, and put them back again. 

If you have a programmable thermostat every brand and model have a different way of resetting. Follow the steps set out in the thermostat’s manual. If you don’t have a manual or can’t find information online you can try to reset the thermostat by pushing the recessed reset button with a pin or paper clip. If you can’t find the recessed button, shut off the circuit breaker to the thermostat and put it on again after 30 seconds.

If the resetting has been successful the thermostat will start working and your HVAC system, including the AC, will start to work again.

Wrong batteries have been used

With all the different types of batteries on the market, it is very easy to buy the wrong batteries for your thermostat. 

Before purchasing new batteries for the thermostat look in your manual or online what voltage the batteries should be. Normally the voltage is also mentioned at the inside of the battery compartment.  The wrong batteries might not be delivering enough voltage to power your thermostat. Generally, you have to use 1.5V AA batteries – but check before you buy. 

If you discover that the batteries are not the correct voltage, replace them with the prescribed batteries and in most instances the thermostat – and your AC – will work again.

Batteries have been inserted incorrectly

It may sound simple, but it is worthwhile to check whether the batteries are fitted correctly. Ensure that the negative and positive ends of the batteries correspond with the polarity indications inside the battery compartment. 

If you haven’t replaced the batteries with new ones in one go, you’ll have to take them out again, get new batteries and insert all of them (normally not more than two) in one go. The thermostat, and thus your AC, might not start working if you’ve mixed new and old batteries.   

An acid leak into the thermostat

If you have a battery thermostat, in other words, a thermostat working solely by batteries and your AC doesn’t work after changing the batteries, open the battery case and take out all the batteries. Inspect the battery contacts. If they are corroded, clean them and put the batteries back again

If the AC is still not working take the batteries out again and look whether you can detect acid marks caused by the previous set of batteries.  If acid has gone into the thermostat you’ll have to call a technician to try and clean it or replace the thermostat.

The wall-mounted thermostat is not making contact

If you have a wall-mounted thermostat and it requires that you have to remove the front part from the fixed wall plate to change the batteries, ensure that it has been put back correctly. If the electrical contacts are not aligned the thermostat and your AC will not work.

Take the top part off again, clean the long pins and align the pins when you put the two halves together again.  Check whether your AC is now working.

There are videos available showing you step by step how to change the batteries of a wall-mounted thermostat. 

The thermostat might have been accidentally set to “heat”

If “my AC is not working” means that it is blowing hot air after the batteries have been changed, check whether the thermostat setting is on “cool” and not on “heat.” It is possible that the setting knob could accidentally be moved to “heat” during the battery changing process. Set the thermostat on “cool” and your AC will blow cool air again.

Conclusion

If your AC has been working well but doesn’t work after you’ve changed the batteries in the thermostat, the problem is normally not with the appliance as such, but with the thermostat. When the thermostat is not working your HVAC appliances cannot function. 

Before you call for technical support, you can troubleshoot the system and try to get the thermostat working again. As there can be various possible reasons why the thermostat is not working after the battery change, there is not a set step-by-step procedure to follow with your troubleshooting. You just have to look at all the possible faults one by one until your thermostat and AC are working again.

If the problem persists, you’ll have to get an HVAC technical expert to repair your system.