Honeywell Thermostat Won’t Turn on AC [Solved]

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If your Honeywell thermostat won’t turn on AC, this could be a sign that your thermostat or AC unit has issues that need to be checked. Several issues could prevent your thermostat from turning on your ac; these include:

  • Low thermostat battery
  • Bad thermostat wiring
  • Tripped breakers
  • Misconfigured thermostat
  • Thermostat compatibility issues
  • Dead relay starter or capacitor
  • Blown out fuses
  • Thermostat that needs replacement
Honeywell thermostat not turning on ac
A man trying to turn on ac

Read also: Honeywell Thermostat Not Cooling [8 Easy Solutions]

9 Reasons Your Honeywell Thermostat Won’t Turn on AC System

Have you been trying to figure out why you can’t turn on an air conditioner with your Honeywell thermostat? Don’t worry, we have possible reasons for that and because we hate to leave you with no solutions, we also included solutions too. So, let’s go into details.

1) Low Batteries (On Selected Models)

Low batteries are the first culprits for compromising your thermostat.  When was the last time you replaced your thermostat batteries? Generally, batteries can stay up to 1 year without replacement.

Honeywell thermostats display a ‘LOW BAT’ warning message about 1-2 months before your batteries die entirely. If you were away from home, you’d have probably missed that message, but this is an easy fix. All you need is to buy a pair of regular AA to replace your old batteries.

Low thermostat batteries can affect how your air conditioner works in the following ways:

  • You’ll have a hard time turning on your ac and turning it off. (Your ac will become insensitive to the turning on and turn off functionality on your thermostat.) So, flat batteries could be the reason your Honeywell thermostat won’t turn on ac unit.
  • Low batteries will cause your ac system not to respond to temperature adjustments on your thermostat. When you try to use some functionality on the thermostat your AC might not respond, this is a good sign that your thermostat needs a new pair of batteries.

How to replace Honeywell thermostat batteries

To change batteries, you don’t need to do anything special on a typical Honeywell thermostat. You’ll need two regular AA batteries. You can buy these batteries in any shop.

Firstly, turn off the thermostat. Then Grasp the first plate of your thermostat and wiggle it straight out to remove it from the wall plate.

You should be able to see batteries behind the first plate. Remove them and replace them with new batteries.

You should do it really quick before your thermostat resets to avoid reprogramming it. When you are done, place it back into the wall plate.

Also read: Honeywell Thermostat Says Heat On But No Heat [Solved]


2) Bad Thermostat Wiring

Bad thermostat wiring could be the reason your Honeywell thermostat is not turning on your ac. Did you install your thermostat yourself? Maybe you missed something during the installation.  Each wire going to your thermostat has its own functions such as heating or cooling.

Open your thermostat, and make sure all wires are connected to the right terminals. There is no standardization on the color code. Because of that, I advise you to check the wiring designation on the control panel of your furnace.

Another great option is to go through the user manual of your HVAC system. For ac functionality, the R and Rc terminal need to be connected separately.

3) Tripped Circuit Breakers

Circuit breakers, if tripped, will cut power to your air conditioner system and it definitely won’t turn on no matter how hard you try to turn it on with your thermostat.  HVAC systems are installed with circuit breakers to protect them when there is a power surge.

Usually, heat pumps have two circuit breakers, one for the outdoor unit (air conditioner breaker) and another for the indoor unit (air handler breaker)

Sometimes these breakers can trip, and they will prevent your unit from turning on.

How to reset tripped circuit breakers

  • Go to the circuit breaker box and look for to switches. One for the inside unit, typically labeled air handler, furnace or FAU (Forced air unit).
  • If the breaker switch is in the middle position, you know it is tripped. To reset it, you need to push it on the other end and then flip it back to its original position.
  • For the outdoor unit breaker, look for a switch labeled air conditioner.

It’s worth noting that breakers don’t just break without a reason, usually, when there is a glitch or surge, they will trip. Resetting a breaker should solve the problem

However, if the breaker trips for a second time immediately, there could be another problem with the equipment. At this point, you need to call HVAC professionals to come and have a look.

4) Misconfigured Thermostat

Did you recently upgrade your thermostat? Sometimes when you have a new thermostat, you might forget to configure it to work with your existing HVAC system. In this case get your user manual and check out how to properly configure your thermostat model. Make Sure its correctly configured for your system and all its stages.

5) Thermostat Compatibility Issues

If your thermostat won’t turn on you air conditioner, you may want to check if its compatible with your existing HVAC system.  Smart thermostats may have compatibility issues with the older HVAC systems.

Before buying a new thermostat, find out from the manufacturer or seller if it is compatible with your HVAC system.

6) Dead Starter Relay Capacitor

A capacitor gives the compressor power to start. And a starter relay sends power from the capacitor to the compressor. If any of the two malfunctions, the compressor won’t start and that means no cooling despite turning on your thermostat.

If you hear any unusual sounds from your outdoor unit, it’s definitely a good idea to call HVAC professionals to come and find out what’s really going.

Both the capacitor and the relay starter are replaceable. In the worst-case scenario you might need to replace your compressor, but that is rare.

7) Blown Out Fuses

After trying out the tips given above, and your ac won’t turn on then check if you have any blown out fuses.

You can find fuses in your furnaces’ control board compartment. If fuses are blown out, it could be why your Honeywell thermostat is not turning on ac. Replacing your fuses should fix the problem.

If there blown-out fuses, they will blow again after replacement. My advice is that if you have blown-out fuses, call an HVAC technician to come and pinpoint the source of the problem.

Thermostat That Needs replacement

Finally, if nothing seems to be helping, perhaps you should consider replacing your thermostat. Programmable thermostats can be convenient because you can schedule the temperature at different times.

Smart thermostats are even more convenient but are not as cost-effective as programmable thermostats. With smart thermostats, you can control the temperature of your home from your devices such as a smart phone or laptop.

Just make sure before you buy a new thermostat its compatible with your HVAC system.

Final Thought 

When your Honeywell thermostat won’t turn on ac, try out the DIY in this guide before calling HVAC professionals.  However, if the problem is not resolved, call Honeywell support for further assistance with your specific model.

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