Nothing runs at all
The thermostat says cooling, but you do not hear the indoor blower or the outdoor condenser.
Start here: Check thermostat mode and setpoint, then check the furnace or air handler switch and the HVAC breakers.
Air Conditioner Troubleshooting
AC Thermostat Says Cooling but Unit Off
Direct answer: When a thermostat says cooling but the air conditioner stays off, the most common causes are a thermostat setup issue, lost power to the indoor or outdoor unit, a clogged filter causing a safety shutdown, or a condensate drain problem tripping a float switch.
Most likely: Start with the easy split: is nothing running at all, or is the indoor blower running while the outdoor unit stays silent? That one detail saves a lot of guessing.
Most of these calls turn out to be a simple control or power problem, not a dead compressor. Reality check: a thermostat can call for cooling even when the AC has been shut down somewhere else. Common wrong move: flipping breakers on and off repeatedly without checking whether one is tripped or whether the drain pan is full.
Don’t start with: Do not start by replacing the thermostat, capacitor, or contactor just because the screen says Cool On.
If the thermostat is blank or dim,check thermostat batteries or HVAC power first.
If the thermostat is lit and says cooling,listen for the indoor blower and then check whether the outdoor condenser is running.
Last reviewed: 2026-04-05
What this usually looks like
Indoor air is moving but outside unit is off
Air comes from the vents, but it is room temperature and the outdoor unit is silent.
Start here: Check the outdoor disconnect, condenser breaker, and whether the thermostat is actually calling for cooling to the outdoor unit.
System stopped after water or a full drain pan
The AC was working, then quit, and you see water near the air handler or in the auxiliary pan.
Start here: Look for a tripped condensate float switch and a clogged drain line before assuming an electrical failure.
Thermostat looks normal but acts out of sync
The display says cooling, but the system starts late, never starts, or behaves differently than the setting.
Start here: Confirm fan is on Auto, replace thermostat batteries if used, and make sure the thermostat is firmly mounted and level on its base.
Most likely causes
1. Thermostat settings or thermostat power issue
A thermostat can display cooling while the fan is set wrong, the batteries are weak, or the thermostat is not making a clean call to the system.
Quick check: Set mode to Cool, fan to Auto, and lower the setpoint at least 3 to 5 degrees below room temperature. If it uses batteries, replace them.
2. Lost power to part of the AC system
It is common for the indoor unit and outdoor unit to have separate power points. One side can be live while the other is dead.
Quick check: Check the HVAC breakers, the furnace or air handler service switch, and the outdoor disconnect near the condenser.
3. Condensate safety switch shut the system down
If the drain line backs up or the pan fills, many systems cut cooling to prevent water damage.
Quick check: Look for standing water in the drain pan, water around the air handler, or a float switch sitting high in the drain assembly.
4. Airflow restriction or iced coil causing a shutdown
A badly clogged air filter can choke airflow, freeze the evaporator coil, and leave the system unable to cool normally.
Quick check: Pull the filter and inspect it in good light. If it is packed with dust or the indoor refrigerant line is icy, stop and let the system thaw.
Step-by-step fix
Step 1: Set the thermostat up for a real cooling call
A lot of no-start complaints are just fan mode, schedule, battery, or mounting issues at the thermostat.
- Set the thermostat to Cool, not Auto changeover or Off.
- Set the fan to Auto, not On, so you can tell whether the system is actually starting a cooling cycle.
- Lower the set temperature 3 to 5 degrees below the room temperature shown on the display.
- If the thermostat uses batteries, install fresh ones.
- Make sure the thermostat face is snapped fully onto its base and the screen is not showing delay, hold, or low battery warnings.
- Wait up to 5 minutes in case the system is in a built-in compressor protection delay.
Next move: If the indoor blower and outdoor unit both start after the delay, the issue was thermostat setup or weak thermostat power. If the thermostat still says cooling and nothing changes, move to power checks next.
What to conclude: The thermostat may be calling correctly, or it may only be displaying a demand without the rest of the system getting power.
Stop if:
- The thermostat wiring is exposed or loose and you would need to handle live low-voltage wiring.
- You smell burning plastic or see discoloration at the thermostat base.
Step 2: Check whether the indoor unit has power
The thermostat and outdoor unit both depend on the furnace or air handler being powered up and ready.
- Go to the indoor unit and confirm the service switch is on. It often looks like a regular light switch near the furnace or air handler.
- Check the main electrical panel for a tripped HVAC breaker. Reset a tripped breaker once only by switching it fully off, then back on.
- Make sure the blower compartment door is fully seated if your system has a door safety switch.
- Listen for any indoor blower sound after restoring power and calling for cooling again.
Next move: If the indoor blower starts and cooling returns, the issue was lost power or an open blower door switch at the indoor unit. If the indoor unit still stays dead, or the breaker trips again, stop there.
What to conclude: No indoor power means the thermostat can look normal while the air handler never responds.
Stop if:
- The breaker trips again after one reset.
- You see scorch marks, melted insulation, or smell hot electrical odor near the indoor unit.
- You would need to remove electrical covers beyond the normal blower door.
Step 3: Separate an indoor-only run from a full system shutdown
If the blower runs but the outdoor condenser does not, you are dealing with a different problem than a total no-power condition.
- Stand near a supply vent and confirm whether air is moving.
- If air is moving, go outside and listen for the condenser fan or compressor hum.
- Check the outdoor disconnect box near the condenser and make sure the disconnect is fully inserted and on.
- Check the condenser breaker in the main panel if the outdoor unit is still silent.
- Look for obvious signs of trouble outside, like a unit that tries to start then clicks off, or a fan that never moves.
Next move: If the outdoor unit starts after restoring disconnect or breaker power, monitor it through a full cooling cycle. If the indoor blower runs but the outdoor unit stays off with power available, the problem is beyond the simple homeowner checks on this page.
Stop if:
- The outdoor breaker trips again.
- The condenser buzzes, clicks, or hums without starting.
- You would need to open the condenser electrical compartment.
Step 4: Check for a condensate drain shutdown or frozen airflow problem
Water backup and airflow restriction are common reasons an AC suddenly stops or seems to ignore the thermostat.
- Inspect the air filter and replace it if it is heavily loaded with dust.
- Look for water in the auxiliary drain pan or around the indoor unit.
- Check whether a condensate float switch is raised or whether the drain line is visibly backed up.
- If you see ice on the refrigerant line or at the indoor coil area, turn cooling off and set the fan to On to help thaw the coil.
- If the drain line is the obvious problem, clear the clog at the service opening or exterior drain termination if that is safely accessible.
Next move: If clearing the drain or replacing a clogged filter lets the system restart normally after thawing, you found the cause. If the float switch keeps tripping, the pan refills, or the coil ices again, the system needs deeper service.
Step 5: Decide between a simple fix and a service call
By this point you should know whether the problem was settings, power, drain safety, filter airflow, or a deeper electrical or refrigerant issue.
- If the system came back after thermostat correction, power restoration, filter replacement, or drain clearing, run it for 15 to 20 minutes and check for steady cool air.
- If the indoor blower runs but the outdoor unit still does not, schedule service instead of replacing condenser parts by guesswork.
- If the system repeatedly freezes, trips breakers, or stops on drain safety, address that exact issue before running it hard again.
- If the thermostat still behaves oddly even with fresh batteries, correct settings, and confirmed HVAC power, a thermostat replacement becomes a reasonable next step.
- If a float switch is visibly damaged or stuck after the drain problem is corrected, replacing that switch is a reasonable next step.
A good result: If the AC now starts, cools steadily, and drains normally, you can move on to verification and prevention.
If not: If the unit still will not run correctly, the safe next move is professional HVAC service.
What to conclude: Simple control and maintenance issues are homeowner territory. Repeated trips, hidden electrical faults, and refrigerant-side problems are not.
Stop if:
- You are considering opening sealed refrigerant components or condenser electrical compartments.
- The system runs only briefly, makes loud electrical noises, or gives off a burning smell.
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FAQ
Why does my thermostat say cooling but nothing is happening?
Usually the thermostat is calling for cooling, but the system is blocked by a power loss, a drain safety switch, a clogged filter and freeze-up, or a thermostat setup problem. The display alone does not prove the whole AC system is actually able to start.
If the blower runs inside but the outside AC unit is off, what does that usually mean?
That usually means the indoor side has power but the outdoor condenser does not, or the condenser has an internal electrical problem. Check the outdoor disconnect and breaker first. If power is present and it still will not start, that is usually a service call.
Can a clogged air filter make it seem like the thermostat is not working?
Yes. A badly clogged filter can reduce airflow enough to freeze the indoor coil or trigger poor cooling behavior that looks like a thermostat problem. It is one of the first things worth checking because it is common and easy to fix.
Will a full drain pan shut the AC off?
On many systems, yes. A condensate float switch can stop cooling when the drain line backs up or the pan fills. That is meant to prevent water damage, so if you see water near the air handler, check the drain problem before chasing electrical parts.
Should I replace the thermostat first?
Not first. Thermostats do fail, but they are not the most common reason for this symptom. Check settings, batteries, indoor power, outdoor power, filter condition, and drain safety first. Replace the thermostat only after those basics are confirmed.
How long should I wait after changing the thermostat setting?
Give it up to 5 minutes. Many systems use a short anti-short-cycle delay to protect the compressor, so the thermostat may say cooling before the equipment actually starts.