Why does my thermostat click but the AC won't turn on?

If the thermostat clicks but the AC doesn't start, the thermostat is working correctly — the problem is on the AC side. Common causes: (1) the outdoor unit breaker is tripped, (2) the AC contactor is stuck or burned out, (3) the capacitor has failed (you may hear a humming from the outdoor unit), (4) the condensate drain pan is full and the float switch has shut off the system. Check the outdoor unit breaker first.

Why is my AC blowing warm air even though the thermostat is set to cool?

Check these in order: (1) Verify the thermostat is set to "Cool" mode, not "Heat" or "Fan Only." (2) Ensure the set temperature is at least 3 degrees BELOW the current room temperature. (3) Check that the outdoor AC unit is running — if the fan isn't spinning, the compressor isn't on. (4) Check for a dirty air filter — a severely clogged filter can freeze the evaporator coil, blocking airflow and cooling.

How long should I wait for the AC to kick on after changing thermostat settings?

Most thermostats have a built-in compressor protection delay of 5 minutes. This prevents the compressor from short-cycling, which can cause damage. After changing settings, wait at least 5 minutes. If the AC doesn't start after 5 minutes, there may be a wiring or equipment issue. Some Honeywell models display "Wait" or "Cool On" (blinking) during this delay period.

Can a bad thermostat cause the AC to freeze up?

Indirectly, yes. If the thermostat causes the AC to short-cycle (turn on and off rapidly), the evaporator coil doesn't have time to defrost normally, leading to ice buildup. Also, if the thermostat's temperature sensor reads too low, it may run the AC continuously, eventually freezing the coil. However, the most common cause of freezing is a dirty air filter or low refrigerant, not the thermostat.

My AC works but the fan doesn't turn off when it reaches temperature. Why?

Check the fan setting on your thermostat — if it's set to "On" instead of "Auto," the fan will run continuously regardless of temperature. Switch to "Auto" mode. If the fan still won't turn off in "Auto" mode, the G wire (fan) may be shorted to the R wire (power), or the fan relay on the furnace board is stuck. Disconnect the G wire at the thermostat and see if the fan stops.

Do I need a special thermostat for AC?

No — virtually all modern thermostats support both heating and cooling. You need a Y wire (yellow) connected from the thermostat to the AC contactor/compressor. If you have central AC, any thermostat labeled "heating and cooling" will work. For heat pump systems, you need a thermostat that supports heat pump mode (O/B terminal) — most smart thermostats and many programmable models support this.

AC Troubleshooting

AC Thermostat Not Working?

Fix cooling problems, warm air issues, and unresponsive AC controls. 80% of "AC not cooling" problems are caused by simple issues you can fix yourself.

Last updated: February 2026

Safety Warning

The outdoor AC condenser unit operates on 240V — never open the electrical panel on the outdoor unit. Always turn off the AC breaker and disconnect box before inspecting any AC components. Thermostat wiring is safe (24V), but AC equipment wiring is dangerous.

What's Your AC Doing?

AC won't turn on at all

Likely cause: Tripped breaker, blown fuse, or dead thermostat

Try steps: 1, 2

Blowing warm/room temp air

Likely cause: Outdoor unit not running, frozen coil, or low refrigerant

Try steps: 2, 3, 8

Fan runs but no cooling

Likely cause: Compressor failure or refrigerant leak

Try steps: 3, 7

Thermostat says "Cool On" blinking

Likely cause: Normal 5-min compressor delay

Try steps: 1

Short-cycling (on/off rapidly)

Likely cause: Dirty filter, frozen coil, or oversized unit

Try steps: 4, 8

AC runs non-stop

Likely cause: Undersized unit, low refrigerant, or thermostat sensor issue

Try steps: 1, 4

8-Step AC Thermostat Troubleshooting

Verify thermostat settings for cooling

Press the "Mode" or "System" button and set to "Cool" (not "Heat," "Auto," or "Off"). Set the target temperature at least 3-5 degrees BELOW the current room temperature displayed. Set the fan to "Auto" (not "On"). If your thermostat shows "Cool On" blinking or "Wait," the compressor delay is active — wait 5 full minutes.

Check both circuit breakers

AC systems use TWO breakers: one for the indoor air handler/furnace and one for the outdoor condenser unit. Check your electrical panel for both — they're often labeled "AC," "Condenser," "Air Handler," or "HVAC." Flip any tripped breakers fully OFF, wait 30 seconds, then back ON. Also check for a disconnect box near the outdoor unit — it contains a pull-out fuse block.

Inspect the outdoor condenser unit

Go outside and check your AC condenser unit. Is the fan spinning? If yes, the compressor is receiving the signal from the thermostat. If no, check for: (1) a tripped disconnect near the unit, (2) a humming sound (could indicate a bad capacitor), (3) debris blocking the fan blade. Clear any leaves or grass from around the unit — there should be 2 feet of clearance on all sides.

Check the air filter

A severely dirty air filter is the #1 cause of AC problems that get blamed on the thermostat. A clogged filter restricts airflow, causing the evaporator coil to freeze and the system to shut down on safety limits. Check your filter — if you can't see light through it, replace it immediately. After replacing, wait 4-6 hours for any ice on the coil to melt before restarting the AC.

Check the condensate drain

Most AC systems have a condensate drain line and a safety float switch. When the drain line clogs (common in humid climates), the float switch shuts off the AC to prevent water damage. Find the drain line — it's usually a PVC pipe near the indoor unit that drips water outside. If it's not dripping during AC operation, it may be clogged. Clear it with a wet/dry vacuum on the outdoor end or flush with vinegar.

Inspect thermostat wiring for cooling

Turn off both HVAC breakers. Remove the thermostat cover. For AC to work, you need at minimum: R wire (red, power) and Y wire (yellow, cooling). Verify the Y wire is securely connected. If your thermostat has separate Rc (cooling) and Rh (heating) terminals, ensure the R wire is connected to Rc (or both are jumpered). Tug each wire gently to confirm connections are solid.

Test with thermostat bypass

With the outdoor unit breaker ON and the indoor unit breaker ON, briefly twist the R and Y wires together at the thermostat (don't touch bare wire to anything else). If the outdoor unit starts, the thermostat is the problem. If nothing happens, the issue is in the wiring run or the outdoor unit itself. Separate the wires immediately after the test (leave connected no more than 2-3 minutes).

Check for frozen evaporator coil

If the AC was running but not cooling, the evaporator coil may be frozen. Open the furnace/air handler access panel and look at the coil. If you see ice, turn off the AC but leave the fan on "On" mode to defrost. It may take 4-24 hours to fully thaw. Once thawed, replace the air filter and restart. If it freezes again, the refrigerant may be low — this requires an HVAC technician.

If these steps resolved your issue, your thermostat should now be working correctly.

Pro Tip

Change your AC filter every 1-3 months during cooling season. A clean filter is the single most effective way to prevent AC problems. Set a phone reminder for the first of every month to check it.

Is It the Thermostat or the AC?

Use this decision tree to determine where the problem is:

1Thermostat screen is blank

Thermostat or power issue

Check batteries, breaker, furnace fuse

2Screen on, clicks when you set to cool

Thermostat is working. AC equipment issue.

Check outdoor unit, breaker, capacitor

3Screen on, NO click when you set to cool

Thermostat may be faulty

Check wiring. Try R+Y wire bypass test.

4R+Y bypass starts the AC

Thermostat is definitely faulty

Replace the thermostat

5R+Y bypass doesn't start AC

Wiring or AC equipment issue

Check wire continuity. Call HVAC tech.

Related Guides

Thermostat Not Working

General troubleshooting

Thermostat Wiring

Y wire & cooling circuit

Fixing Nest Thermostat

Nest-specific AC issues

Fixing Honeywell

Honeywell AC settings

Frequently Asked Questions

Written by

ThermostatFixer Editorial Team

Our team of HVAC enthusiasts and DIY experts creates detailed thermostat troubleshooting guides, wiring diagrams, and repair tips to help homeowners fix common thermostat issues without calling a technician.