Thermostat Set to Cool but Nothing Happens

If nothing happens after you set the thermostat to Cool, start with the thermostat and the normal switches. This AC troubleshooting hub covers what to check before you suspect a component failure.

1. The thermostat batteries are depleted

The thermostat batteries are depleted

The thermostat screen is blank or shows a low-battery warning.

Difficulty

Easy

Time

5 minutes

Tools

replacement-batteries

Trane says to check batteries when the thermostat screen is blank. Replace them only if the thermostat’s manual describes ordinary user-replaceable batteries. Do not remove the thermostat from the wall or touch its wiring.

If the display returns but the AC still does not respond, check the mode and program. A working display does not prove that the thermostat is sending a cooling call.

2. The mode or setpoint is wrong

The mode or setpoint is wrong

The control is not on Cool or the target is not below room temperature.

Difficulty

Easy

Time

5 minutes

Tools

None listed

Trane says to choose Cool, set the fan to Auto and set the target below the indoor temperature. Carrier also lists the Cool setting and fan control among its first checks. A mode left on Heat, Off or fan-only will not request normal cooling.

Wait a few minutes after a change and check whether the system responds. Do not keep lowering the target if the unit remains idle. If the display looks right, a schedule or power switch may be the next clue.

3. A schedule overrides the setting

A schedule overrides the setting

A programmed period changes the target after a manual adjustment.

Difficulty

Easy

Time

5 minutes

Tools

None listed

Carrier says to check whether a schedule has changed the target. A manual adjustment can be temporary on a programmed thermostat. Review the active period using the normal interface and note whether the target changes again.

The cited pages do not give one menu sequence for every thermostat. Use its model manual to adjust or pause the schedule; do not reset the whole control unless the manual tells you to.

4. A normal power switch is off

A normal power switch is off

A clearly labeled indoor equipment switch is visibly off.

Difficulty

Easy

Time

5 minutes

Tools

None listed

Trane lists an indoor shutoff switch near the furnace or air handler. It can resemble a light switch. Look for a clearly labeled, accessible control; do not open a panel or move the equipment.

If the switch is damaged, wet or not labeled, leave it alone. A unit that still does not respond after the correct control is on needs more diagnosis.

5. A breaker has tripped

A breaker has tripped

A visible breaker indicator or position shows a trip.

Difficulty

Moderate

Time

5 minutes

Tools

None listed

The sources differ. Trane says to contact an HVAC professional and not flip a tripped breaker. Carrier describes an AC reset, but says not to try again if the breaker trips immediately afterward. Do not repeat resets or open an electrical panel.

Tell the technician which circuit appears tripped and whether the unit stopped suddenly. A recurring trip can point to an electrical fault that needs professional testing.

6. A safety switch or frozen coil stopped the system

A safety switch or frozen coil stopped the system

There is water at the indoor unit, visible ice or a system shutdown.

Difficulty

Pro

Time

60 minutes

Tools

None listed

Trane lists a clogged condensate drain and frozen coil among reasons an AC may not turn on. Look for water around the indoor equipment or visible ice without removing an access panel. Do not bypass a condensate safety switch.

If there is water, use the AC leak guide. If ice is visible, follow the frozen-coil guidance and leave cooling off. Call if the unit remains stopped or the cause is not obvious.

7. The thermostat or control needs diagnosis

The thermostat or control needs diagnosis

Settings and visible switches are correct but no cooling call starts.

Difficulty

Pro

Time

60 minutes

Tools

None listed

Part cost

$150–$500Source: AC Repair Cost (Fixr) · Checked October 12, 2026

Trane says that if the AC still will not come on after the thermostat and other checks, the thermostat may not be the problem. The fault can be elsewhere in the system. Do not replace the thermostat based on a non-start alone.

Fixr lists a thermostat replacement range below. It is a secondary estimate, not proof that the thermostat failed. Ask the technician to identify the cause before comparing the quote with the AC repair-cost guide.

$150–$500Source: AC Repair Cost (Fixr) · Checked October 12, 2026

Two kinds of thermostat, two kinds of power problem

A thermostat that shows a display but does nothing, and one that shows nothing at all, are different faults. The thermostat makers describe both, and their pages explain why a thermostat that “has power” can still fail to start the AC.

Battery-powered and wired thermostats

Honeywell Home’s support page on a blank display gives a short path. If the thermostat is battery powered, check the batteries and replace them if needed. If it is wired to a common wire, try powering it with batteries; if it still will not power up, the page says you may need to involve a technician. If batteries power a common-wired unit, confirm the system itself has power by checking the breakers, the furnace power switch and the furnace door kill switch. If the system has power but the thermostat runs only on batteries, again involve a technician. The point to take from it: a blank screen can be a battery, a wiring problem or a system with no power, and you can sort the first and third without touching a wire.

Trane’s page agrees on the first step: if the screen is blank, replace the batteries. It then says to set the mode to Cool and the fan to Auto, since settings may have been changed, and to set the target temperature below the current indoor temperature. Lennox says the same with a number: set it to Cool and at least 3 to 5 degrees below room temperature, and wait a few minutes before judging the result.

Why some smart thermostats go dark when the AC stops

Google’s Nest help page explains a failure that many owners do not know about. A Nest thermostat usually powers itself through the Y1 wire, the air conditioner wire, instead of a common wire. If the AC stops sending that power, the thermostat shows an error. Google says common causes are system breakdowns or maintenance needs, and low-pressure or safety cutout switches that shut off the AC in extreme cold, and that an HVAC professional should diagnose the root cause. For the power-loss codes (E10, E30, E66, E80, E116, E160, E196 and M13), Google describes turning off power at the main breaker, removing the display, photographing the wiring, removing the Y wires and taping the ends, restoring power and charging or replacing the batteries. It adds that cooling stays off until the Y1 wire is reconnected, so a common wire should be installed to restore it. For the battery codes (E296, N262, N263, N412, N413), the step is to check the battery level and replace the batteries if necessary.

This guide does not tell you to remove a thermostat or handle its wires, and Google’s steps are written for people who are comfortable doing so. Use them as a way to recognize the situation. If your smart thermostat shows one of these codes and the AC has not cooled, say so when you call: it points to a thermostat power problem and, behind it, possibly a shutdown by the AC’s own safety switches. For prevention, Google recommends installing a common wire or a power accessory such as the Nest Power Connector, through a professional if needed, and notes an unused thermostat wire may already work as one.

The checks that need no tools, in order

1. Look at the display. Blank means batteries or power; lit means a settings or schedule issue.
2. Set the mode to Cool and the fan to Auto. Set the target 3 to 5 degrees below room temperature. Wait a few minutes.
3. Check for a hold or schedule that overrides the setpoint.
4. If the thermostat is blank, replace the batteries only if it uses ordinary ones.
5. Check the indoor shutoff switch, which Trane says can look like a light switch near the furnace, and the breaker panel for a tripped indoor or outdoor breaker.
6. Look for water around the indoor unit. Trane says a clogged drain can shut the system down when a condensate safety switch is installed, and Lennox lists a tripped float switch among causes of an outdoor unit that will not turn on.
7. Call for service if the thermostat is lit and calling for cooling and the outdoor unit still does not start, or if a breaker trips again.

Trane’s warning about the breaker is worth repeating: do not flip it, because a trip may signal a serious system problem. If the unit is under warranty, doing your own repairs could void it.

Before the call

Record the thermostat display, selected mode, target temperature, whether the indoor fan runs, and what you can see at the normal switch or breaker. Note any water or ice. Do not touch a wire, capacitor or refrigerant line.

Carrier describes a reset that starts with the thermostat off and the AC breaker off for at least 30 minutes. Its separate cooling page gives a shorter power-off interval of about five minutes. Trane’s advice is not to flip a tripped breaker. Because those instructions differ, follow the model manual and do not use a reset when the breaker trips or the equipment shows a fault.

If the outdoor unit runs but the air remains warm, use the warm-air diagnosis. If it is silent, use the outdoor unit guide. Those symptom paths are more useful than replacing a thermostat without a diagnosis.

Frequently asked questions

Why does nothing happen when my thermostat is set to Cool?

Check batteries, Cool mode, the setpoint, schedule and a normal labeled switch. If those are correct, the problem may be elsewhere in the system.

Should I reset an AC breaker?

Trane says not to flip a tripped breaker and to contact an HVAC professional. Carrier says not to retry if it trips immediately after resetting.

How much does an AC thermostat replacement cost?

Fixr's cited estimate appears below as a dated planning range. It is not a diagnosis or a quote for your home.

Sources

• AC Not Turning OnTrane
• Troubleshoot an AC Not WorkingCarrier
• Gas Furnace TroubleshootingBryant
• Air Conditioner Repair CostFixr Editorial Team (secondary)
• How do I fix a blank display on my TH6320ZW thermostat?Honeywell Home Support
• Troubleshoot E10, E30, E66, E80, E116, E160, E196, E296, M13, N262, N263, N412 and N413 help codesGoogle Nest Help
• Outside AC Unit Not Working?Lennox