Picture of the Honeywell RTH9580WF Internet Thermostat, being snapped onto wall plate.

Honeywell Thermostat Not Turning ON AC

The Honeywell thermostat not turning ON AC can happen for many reasons. A faulty thermostat, a broken heating system, power losses, damaged wiring, et al. Here, we list some of these causes, and offer repair tips to solve these no-heat issues. First, we look at what might have happened with the Honeywell Thermostat itself. Then we explore the furnace issues that can also cause no heating at all.  Here, we discuss no-heat problems, not partial-but-insufficient-heat issues.  These address the case when you get no heating at all. But they do not cover the cases where you get some yet not enough heat.

Honeywell Thermostat Not Turning ON AC: Possible Causes

Summary of Causes

    1. The temperature offset setting is too large.
    2. Power failure keeping air conditioning from coming ON.
    3. The thermostat is not level on the wall.
    4. Temperature set too high.
    5. Poor electrical connections.
    6. Mechanical thermostats can bind up.
    7. Thermostat not set to correct operational mode (heating or cooling mode).

Thermostat Temperature Offset Set Too High


The thermostat temperature display seems to show that the current room temperature never reaches the set cooling temperature. Yet the AC never turns on. So in this case, the display offsets may be too high.  That is, the thermostat  acts as if the current indoor temperature is lower than it really is. And when the t-stat thinks that the room is cooler, it will not turn the AC ON.

Consider this.

    • Let’s say that you set the thermostat to cool to 74 degrees.
    • Further, you have set your display temperature offset to -3 degrees. This makes your thermostat display a current temperature reading that is three degrees cooler than the environment actually is. In this case, when the current house temperature is 74 degrees, the thermostat would show 71 degrees.

Now the thermostat calls for AC when the temperature it reads rises above 74 degrees.  But a -3 degree offset means that the actual temp must go up 3 degrees above 74. So though you set the temp to 74, you won’t get AC until the actual room temp goes above 77.  Now in apartment buildings, a room on the lower floors might easily hold at 74 degrees. So you may rarely or never feel the AC working if you set the thermostat to 74.


Decrease the display offset temperature. This helps assure that the temp reading on the thermostat shows as closer to actual room temp.  Plus, an accurate display reading helps prevent the situation where the AC does not come on enough. We suggest that you avoid setting the temperature offset to more than one degree either way (+ or – offset).

Or, just lower the set temperature.  In this scenario, let’s say you want your place to cool to 74 degrees. But with the -3 degree offset, you’ll need to turn the thermostat lower than the temperature you desire.  So you’ll have to set it to 71 degrees to get that 74 degrees actual temperature.

Power Failure Keeps AC from Coming On

Picture of the Honeywell RTH9580WF Internet Thermostat, mounted but powered down, snapped onto wall plate.
Honeywell RTH9580WF Internet Thermostat, mounted but powered down, snapped onto wall plate.


The screen on Honeywell smart thermostats looks blank or dark. No readings or status messages appear. Further, the AC does not run. Plus, you hear no humming at the furnace. But for non intelligent thermostats you may no power loss indication, except that their backlights don’t glow. Yet the effect is still the same, no AC turning on. That is, that the HVAC never runs, no matter the thermostat setting.


Honeywell thermostats often get power right from the furnace system. So, check that all furnace systems are getting electricity. Often there’s a single circuit breaker that feeds the furnace. Then there’s another that feeds the outside compressor unit in HVAC arrangements. So check that all these breakers are ON.

Furthermore, the heating and cooling subsystems often get electricity from separate circuit breakers. So, the thermostat could glean power from either one, although usually not both. A power outage in either system could cause this problem. By checking that the AC control circuits have power, you rule out this cause of thermostat power loss. In this way, a faulty cooling system can make the heating system fail too.

Thermostat Has Poor Wiring or Installation

Picture of the t-stat model RTH9580WF wall plate, mounted, but with wires not yet connected.
The model RTH9580WF t-stat wall plate mounted, but with wires not yet connected.


If the AC problem happened when you installed a new thermostat, you may have wired it the wrong way. Perhaps you reversed the W and Y wires. This would run the cooling stage though the thermostat is actually calling for warmth. Other symptoms may arise too. These depend on which wires and how many of them you connected to the wrong thermostat terminals.


Check for correct wiring at both ends of the thermostat cable. At the thermostat end, securely attach all wires to the right screw terminals in the mounting plate. Then, do likewise at the furnace. Or seek help from a professional heating technician if you feel insecure matching these wires up with the right terminals.

Thermostat Not Level May Cause Honeywell Thermostat Not Turning ON AC


Poor leveling on older thermostats can affect how well they respond to room temperature changes. Those early models used a bi-metal spring to sense temperature. Manufacturers would attach a mercury switch to the outside end of that spring. Then, as room temperature rose and fell, the spring changed size. This caused its outer end to move back and forth along with the temperature changes. This in turn tipped the mercury switch ON or OFF, to bring room temperature nearer to the set temperature.

You needed proper leveling for this tipping to occur as designed. Indeed, if tilted too far off of plum or the horizontal, the cooling system might never turn on. In this case, the mercury inside does not make the contacts as it should when the room gets too cold.


Now you may still have a mercury and bi-metal thermostat. If so, then you should replace it with a current all-electronic model that has no moving parts. Why? Because today’s thermostats have more accurate sensors and many more features.

Plus, modern thermostats work well, no matter how far off of level you install them. They’ll turn on the heat correctly in the house, no matter their position.  Mount them on their sides or upside down. Or, let them dangle by the wires (although we do not recommend this).  But the point is that they will regulate your heat well, in any position.

If you cannot upgrade from an older thermostat, then try drilling new mounting holes. To do that, remove the screws and wires if they get in the way. Then re position the thermostat over the new holes. Finally, drive the screws into those new holes. Use wall anchors in these holes for a stronger hold.

If your t-stat is not too far out of level, try loosening the screws that hold it to the wall. Then, slide the unit toward level. Then, mark the wall with an erasable pencil.  Next, while holding the thermostat in that marked position, tighten the screws again.  Finally, use an eraser to clean off any visible wall markings.

Thermostat Temperature Set Too High

Picture of The temperature at 69 degrees on the thermostat.
The temperature at 69 degrees.


You can set thermostats way higher than the ambient temperature in your home.  During moderate days outside for instance, you might set the cooling temperature to 74 degrees.  But if you live in a basement for example, your apartment may never go above 74 degrees.  Thus, your AC may never come on.


    • If you need more AC, try setting the thermostat to a lower temperature.
    • Or just be happy your AC is not running any more than it is. Especially if paying your own utility bills.

Bad Electrical Connections Might Cause Honeywell Thermostat Not Turning ON AC

Picture of the Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminal connection points.
Honeywell T8775C1005(2) thermostat wall plate, front view, showing screw terminal connection points.


The solid copper wires in the thermostat cable can break if flexed or bent too much. Or, maybe the installer stapled the cable with too much pressure, and shorted out the individual conductors in that cable.

This issue normally appears during new thermostat testing only. But sometimes, this problem may not show up for decades. Plus, settling of wall beams can flex these stapled wires. So, after many years, this flexing can wear wire coverings, causing faulty or no heating.


Replace the cable between the thermostat and furnace. Avoid tightly stapling the wire, and don’t fasten it more than you must for a secure installation. Lay it loosely.

Mechanical Thermostats can Bind Up


Now and then with smart thermostats, a snap of static electricity near the thermostat, can freeze the touch screen. Also, a power surge in the home’s electrical system can do this too. Also, the data onscreen no longer updates. Further, the AC system may not come on. No matter how far from set temperature the room temperature is, the AC may not run at all.


Picture of snapping a Honeywell Thermostat onto the wall plate.
Snapping a Honeywell Thermostat onto the wall plate.


When this no heat problem happens, try rebooting the thermostat. Turn off its power for ten to twenty seconds. Then switch it on again. This power cycling is a literal snap with many thermostat models. Why? Because they can detach from furnace power without having to unscrew them. Just unsnap them from wall plates. Then, wait a few seconds. And finally snap them back into place to reboot. Or, if your t-stat does not come off of the wall easily, power cycle it at the breaker box. Doing this reboots the thermostat.

Not Set to Desired Operational Mode (Heating or Cooling Mode)

Picture of the t-stat, showing its -System Heat / Cool Mode Setting- screen, with the mode set to OFF.
The t-stat, showing its -System Heat / Cool Mode Setting -screen, with the mode set to OFF.


Certainly you might see a Honeywell thermostat not turning ON AC, when it’s not currently n cooling mode!  On our thermostat,  we can set it to either heating mode, cooling mode, or OFF.  If OFF, you won’t get heat.  Nor will any cool air flow from your registers in Cool mode.


Set the t-stat’s operational mode to Cool.  On our t-stat, we do this in the System tab. Just tap the Cool option and press Done to make the new setting effective.  You should soon thereafter feel some AC.

Other Honeywell Thermostat Posts

    1. Reset WiFi on Honeywell Thermostat — Explained
    2. Honeywell Thermostat Won’t Turn On Heat
    3. How to Unlock Honeywell Thermostat
    4. Honeywell Thermostat Not Turning On Heat
    5. Honeywell Thermostat Restart Instructions

References for Honeywell Thermostat Not Turning ON AC

    1. Official Honeywell Thermostat Support Page
    2. Where to Buy the Honeywell Thermostat