Nothing runs at all
Thermostat is on Cool, but you do not hear the indoor unit or the outdoor unit start.
Start here: Check thermostat mode, setpoint, batteries if used, and both HVAC breakers before anything else.
Indoor AC fan not starting
Air Conditioner Inside Unit Not Running
Direct answer: When the air conditioner inside unit is not running, the usual causes are a thermostat setting issue, a clogged filter causing a freeze-up or shutdown, a condensate safety switch stopping the system, or lost power to the indoor air handler. If the thermostat is calling for cooling but the indoor blower stays dead, the problem moves toward the air handler power supply, blower section, or controls.
Most likely: Start with the easy tells: thermostat set to Cool with the fan on Auto, a badly plugged air filter, water in the drain pan, and any tripped breaker or service switch near the indoor unit.
First separate whether the whole system is dead or just the indoor blower. If the outdoor unit runs but nothing moves inside, shut the system off and check for ice, a soaked drain pan, and indoor power loss before you go any deeper. Reality check: a lot of 'bad blower' calls turn out to be a wet float switch or a filter packed solid. Common wrong move: resetting breakers over and over without finding out why they tripped.
Don’t start with: Do not start by replacing the thermostat, blower motor, or control parts. On this symptom, a drain safety trip or simple power issue is more common than a failed major component.
If you see standing water near the indoor unit,stop cooling and check the condensate pan and drain safety switch first.
If the thermostat clicks but the inside unit stays silent,check the indoor breaker, service switch, and filter before assuming a failed motor.
Last reviewed: 2026-04-04
What this usually looks like
Outdoor unit runs but indoor unit does not
You hear the outside condenser, but there is little or no air from the vents and the inside unit is quiet.
Start here: Turn cooling off and look for a clogged filter, ice at the indoor coil area, or a condensate safety shutdown.
Indoor unit hums or clicks but will not blow
The air handler makes a short hum, buzz, or click, but the blower never gets up to speed.
Start here: Shut power off to the indoor unit and stop there if you are not comfortable around electrical compartments. This points more toward blower or control trouble.
It quit after water or heavy humidity
The system was cooling, then stopped, and you find water in the pan, around the unit, or near the drain line.
Start here: Check for a full drain pan or tripped condensate float switch before chasing electrical parts.
Most likely causes
1. Thermostat setting or call problem
A bumped mode, wrong fan setting, dead batteries, or a thermostat not actually calling for cooling can make the indoor unit look dead.
Quick check: Set the thermostat to Cool, lower the setpoint at least 3 to 5 degrees, and leave the fan on Auto first, then try Fan On to see whether the blower responds.
2. Clogged air filter or frozen indoor coil
A packed filter can choke airflow, let the coil ice up, and leave you with little airflow or a blower that seems not to be running normally.
Quick check: Pull the air filter and inspect it in good light. If it is gray and matted over, replace it. Look for frost or ice at the indoor coil cabinet or refrigerant line.
3. Condensate drain backup tripping the float switch
Many systems shut the indoor unit down when the drain pan fills or the condensate line backs up, especially in humid weather.
Quick check: Look for water in the auxiliary pan, around the air handler, or at the drain outlet. A raised float or wet pan is a strong clue.
4. Lost power to the indoor air handler
A tripped breaker, pulled service disconnect, or switched-off furnace or air handler service switch will stop the inside unit even if the thermostat is calling.
Quick check: Check the indoor HVAC breaker and the light-switch-style service switch mounted on or near the air handler or furnace.
Step-by-step fix
Step 1: Make sure the thermostat is actually calling for the indoor unit
A lot of no-run complaints start with a thermostat that is not really sending a cooling or fan call.
- Set the thermostat to Cool and lower the temperature setting at least 3 to 5 degrees below room temperature.
- Leave the fan on Auto first. Wait a full minute for the system to respond.
- Then switch the fan from Auto to On and listen for the indoor blower.
- If your thermostat uses batteries, replace them if the display is dim, blank, or acting erratic.
- If the thermostat is blank, check for a tripped breaker before assuming the thermostat failed.
Next move: If the blower starts on Fan On or the system starts cooling normally, the issue was likely a setting problem, weak thermostat batteries, or a temporary control glitch. If the thermostat appears normal but the indoor unit stays silent, move to airflow, drain, and power checks.
What to conclude: You are confirming whether the problem is a simple control call issue or a real no-run condition at the indoor unit.
Stop if:
- The thermostat is blank and breakers are tripped repeatedly.
- You smell burning plastic, hot wiring, or see sparking at the thermostat or air handler.
- The thermostat wiring is loose, damaged, or exposed and you are not comfortable working around low-voltage controls.
Step 2: Check the filter and look for signs of a frozen indoor coil
Restricted airflow is one of the most common reasons an AC stops moving air properly or shuts down after running awhile.
- Turn the system off at the thermostat before opening the filter slot or blower door area.
- Remove the air filter and inspect it. If it is heavily loaded with dust, replace it with the same size and airflow rating style the system was using.
- Look for frost or ice on the larger insulated refrigerant line near the indoor unit, at the coil cabinet seams, or around the blower compartment area if visible.
- If you find ice, leave the system off for cooling and switch the fan to On only if the blower still works. Let it thaw fully before restarting cooling.
- Do not chip at ice or pry on coil fins.
Next move: If a new filter and a full thaw restore normal airflow, the immediate problem was likely airflow restriction or freeze-up. If the filter was clean or the blower still will not run after thawing, keep going. The shutdown may be from condensate protection or indoor power loss.
What to conclude: A dirty filter is the easy fix. Ice points to an airflow or cooling problem, but you still need the indoor blower running before judging the rest of the system.
Stop if:
- There is heavy ice buildup inside the cabinet or water is pouring out as it thaws.
- The blower door must be removed near exposed wiring and you are not comfortable shutting power off first.
- You suspect the outdoor unit kept running while the indoor coil was frozen solid.
Step 3: Look for a condensate drain backup and float switch shutdown
On many systems, a backed-up drain line or full pan will stop the indoor unit to prevent water damage.
- With cooling off, inspect the area around the indoor unit for water stains, a wet floor, or a full auxiliary drain pan.
- Look at the condensate drain line and pan area for standing water or a raised float switch if one is visible.
- If the drain outlet is accessible, check whether it is dripping normally or obviously backed up.
- If you can safely clear a simple visible clog at the drain opening, use gentle suction or flush only where the line is meant to be serviced. Do not force water into the unit.
- Once the pan is dry and the float drops back to normal, restore cooling and see whether the indoor unit starts.
Next move: If the indoor unit starts after the pan drains and the float resets, the shutdown was likely caused by a clogged condensate line or pan overflow. If the pan is dry and the float is not tripped, or the unit still does not run, check indoor power next.
Stop if:
- Water has reached wiring, insulation, or ceiling materials around the unit.
- The drain line is hidden in walls or ceilings and you cannot tell where the blockage is.
- The float switch wiring is damaged or you would need to bypass a safety device to keep testing.
Step 4: Check for lost power at the indoor air handler
The indoor unit can lose power even when the thermostat looks normal and the outdoor unit still tries to run.
- Turn the thermostat off before touching any service switch or breaker.
- Check the HVAC breakers in the main panel. Reset a tripped breaker once only by switching it fully off, then back on.
- Find the service switch on or near the indoor air handler or furnace and make sure it is on.
- Make sure the blower door or access panel is seated properly if your unit has a door safety switch.
- Restore the thermostat to Cool and test again.
Next move: If the indoor unit starts after a breaker reset, service switch correction, or properly seated panel, you found the immediate cause. If the breaker trips again, the blower only hums, or the unit stays dead with confirmed power available, stop DIY and schedule service.
Stop if:
- A breaker trips again after one reset.
- You hear loud humming from the indoor unit without the blower starting.
- You would need to open an electrical compartment with live wiring to continue.
Step 5: Decide the next move based on what you found
At this point the safe homeowner fixes are usually done, and the remaining causes are more technical and higher risk.
- If the system now runs, let it complete a full cooling cycle and watch for water, ice, or a breaker trip returning.
- If the indoor blower runs but the air is not cold, move to the warm-air symptom path rather than replacing indoor parts blindly.
- If the outdoor unit runs but the indoor blower stays dead, report that exact symptom when you call for service.
- If the breaker trips again, leave the system off and do not keep resetting it.
- If a drain backup caused the shutdown, clean the line fully and keep an eye on the pan over the next day or two.
A good result: If cooling returns and stays stable, your repair path was likely filter, drain, or power related.
If not: If the indoor unit still will not run after these checks, the remaining likely causes are inside the air handler and should be diagnosed by a pro.
What to conclude: You have ruled out the common homeowner-level causes and narrowed the call down to a much more useful service diagnosis.
Stop if:
- There is any burning smell, smoke, or scorched wiring smell from the indoor unit.
- The system has repeated icing, repeated breaker trips, or water leaking into finished spaces.
- You are tempted to bypass a float switch, door switch, or other safety just to make it run.
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FAQ
Why is my outside AC unit running but the inside unit is not?
That usually points to an indoor-side problem such as a clogged filter and frozen coil, a condensate float switch shutdown, lost power to the air handler, or a blower section fault. Start with filter, drain pan, service switch, and breaker checks.
Can a dirty filter make the inside AC unit stop running?
Yes. A badly clogged filter can choke airflow enough to freeze the indoor coil or make the system shut down abnormally. It is one of the first things worth checking because it is common and easy to fix.
How do I know if the condensate float switch shut my AC down?
Look for water in the drain pan, around the indoor unit, or at the drain line. If the pan is full and the float is raised, that is a strong sign the safety switch stopped the system to prevent overflow.
Should I keep resetting the breaker if the indoor unit will not start?
No. Reset it once only. If it trips again, leave the system off. Repeated breaker trips usually mean a motor, wiring, or control problem that needs proper diagnosis.
What if the indoor blower starts but the air still is not cold?
That is a different symptom path. Once the blower is running, move your diagnosis toward cooling performance rather than no-run causes. Check the warm-air or not-cooling symptom page instead of replacing indoor blower parts blindly.