Why did my AC stop working suddenly?

Sudden total stoppage most often traces to a tripped breaker, a tripped condensate float switch, or a blown low-voltage fuse.

How many times can I reset a tripped breaker?

Once. A breaker that trips again is doing its job — repeated resets on a fault are a fire risk.

Why is my thermostat blank?

Dead batteries, a blown 24V fuse on the control board, or a safety switch that has opened the control circuit.

Is there a reset button on my AC?

Some units have one on the outdoor cabinet or air handler. Pressing it once is fine; if the system trips again, it needs diagnosis.

Why Is My AC Not Working?

Causes, safe checks and when to call a professional

Expert Answer

Start by classifying the failure, because "not working" covers four different problems with four different fixes. Is the thermostat display blank? That is a power or control problem. Does nothing at all happen when you lower the setpoint? Power, thermostat or control wiring. Does the indoor unit run but the outdoor unit stay silent? That is an outdoor electrical fault. Does everything run but the air is not cold? That is a refrigeration problem. Check the thermostat batteries, both breakers and the air handler service switch first — those three account for a surprising share of no-cooling calls.

Check these 3 things first

1Look at the thermostat display for a blank screen versus a lit one.
2Listen for the indoor blower running when you drop the setpoint.
3Walk outside and check whether the condenser is running at all.

YELLOW — Inspection Recommended

How Serious Is This?

GREEN
Usually Manageable
Often resolved with basic homeowner checks and routine maintenance.
YELLOW — this issue's severity level
Inspection Recommended(this issue)
Worth a professional diagnosis soon to avoid a larger repair.
RED
Potentially Serious
Stop using the system and get professional help right away.

Most Likely Causes

1. Tripped breaker or blown fuse
Systems typically have separate breakers for the indoor and outdoor equipment; either can trip independently.
2. Thermostat power or failure
Dead batteries, a tripped condensate float switch cutting the 24V circuit, or a failed thermostat leaves the equipment with no call.
3. Air handler service switch turned off
The switch beside the indoor unit looks like a light switch and is easily switched off during other work.
4. Blown low-voltage fuse or failed transformer
A short in the thermostat wiring commonly blows the control-board fuse, killing the whole system.
5. Condensate float switch tripped
A full drain pan opens the control circuit and shuts the system down deliberately.
6. Failed capacitor or contactor outdoors
The blower runs, the compressor does not, and the house never cools.
7. Compressor or motor failure
Hard component failures stop the system entirely and require diagnosis.

Safe Homeowner Checks

Check the thermostat display — blank means a power or control-circuit problem.
Replace thermostat batteries.
Check both AC-related breakers in the panel; reset once only.
Verify the air handler service switch is ON.
Check the outdoor disconnect pull-out is seated.
Look for standing water in the drain pan that could have tripped a float switch.
Set the thermostat to COOL and lower the setpoint five degrees, then listen for equipment.

Stop and get help if you notice

• Breaker that trips again after reset — stop and call for service.
• Burning smell at the indoor or outdoor unit.
• Buzzing or crackling from electrical equipment.

2-Minute Tests You Can Run

Each test isolates one specific cause so you know what you are dealing with before anyone quotes you a repair. Stop immediately if anything feels unsafe.

Breaker panel check
About 5 minutesTools: None
Tests for: Tripped breaker or blown fuse
1. Open the electrical panel and find the AC breakers.
2. Look for a breaker sitting in a middle, off-tilted position.
3. Switch it fully off, then fully on, one time only.
4. Return to the thermostat and start a cooling call.
Normal result
The breaker stays firmly in the on position and equipment runs.
Problem result
The breaker trips again within seconds of resetting.
What it means: A breaker that re-trips indicates a genuine electrical fault, not a nuisance trip, and needs a technician rather than another reset.
Thermostat display test
About 3 minutesTools: Fresh AA or AAA batteries
Tests for: Thermostat power or failure
1. Check whether the thermostat screen shows anything.
2. Replace the batteries even if the screen looks lit.
3. Confirm the mode is set to COOL, not OFF or HEAT.
4. Lower the setpoint five degrees below room temperature.
Normal result
The display lights up and shows a cooling call or a countdown.
Problem result
The screen stays blank after fresh batteries.
What it means: A blank screen after new batteries points to a blown control fuse, a dead transformer, or an open safety switch feeding the thermostat.
Drain pan inspection
About 5 minutesTools: Flashlight
Tests for: Condensate float switch tripped
1. Locate the indoor air handler and find the secondary drain pan beneath it.
2. Shine a light into the pan and look for standing water.
3. Check the small float switch clipped to the pan or the drain line.
4. If water is present, note it and plan to clear the drain line.
Normal result
The pan is dry and the float switch sits low.
Problem result
Water sits in the pan and the float has risen.
What it means: A full pan has deliberately opened the control circuit to prevent overflow, and the system will not run until the water is cleared.
Service switch verification
About 2 minutesTools: None
Tests for: Air handler service switch turned off
1. Find the switch mounted on or near the indoor air handler cabinet.
2. Confirm it is flipped to the ON position.
3. Flip it off and on once if it looks ambiguous.
Normal result
The switch is up and firmly in the on position.
Problem result
The switch was down or loose.
What it means: This switch is often bumped or intentionally shut off during attic or closet work and simply forgotten.

Want a pro to confirm it?

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What This Typically Costs

Thermostat batteries
DIY
$5-$15
Professional
N/A
Typical time
5 minutes
Air filter replacement
DIY
$15-$40
Professional
Included in maintenance visits
Typical time
10 minutes
Thermostat replacement
DIY
$25-$150
Professional
$150-$350 installed
Typical time
30-60 minutes
Capacitor replacement
DIY
Not a DIY repair
Professional
$150-$400
Typical time
30-60 minutes
Contactor replacement
DIY
Not a DIY repair
Professional
$150-$350
Typical time
30-60 minutes
Control board or transformer
DIY
Not a DIY repair
Professional
$250-$650
Typical time
1-2 hours

Typical repair costs and time
Problem	DIY cost	Professional cost	Typical time
Thermostat batteries	$5-$15	N/A	5 minutes
Air filter replacement	$15-$40	Included in maintenance visits	10 minutes
Thermostat replacement	$25-$150	$150-$350 installed	30-60 minutes
Capacitor replacement	Not a DIY repair	$150-$400	30-60 minutes
Contactor replacement	Not a DIY repair	$150-$350	30-60 minutes
Control board or transformer	Not a DIY repair	$250-$650	1-2 hours

Ranges are national estimates for planning only. Local labor rates, equipment access and parts availability change what you will actually be quoted.

Working Through It

Does the thermostat have a display?
Yes: Control power exists — continue.
No: Check batteries, the float switch and the low-voltage fuse or transformer.
Does the indoor blower run when you call for cooling?
Yes: Indoor power is fine — focus on the outdoor unit.
No: Check the air handler breaker and service switch.
Does the outdoor unit run?
Yes: Both halves run — this is a refrigeration or airflow performance problem.
No: Check the outdoor breaker and disconnect, then call for electrical diagnosis.

What the technician will check

Line and low-voltage power verification.
Control board, fuse and transformer testing.
Float switch and safety circuit continuity.
Capacitor, contactor and motor testing.
Refrigerant charge if the system runs without cooling.

Repair considerations

• Fuses, batteries, float switches and capacitors are inexpensive repairs.
• Control boards and ECM motors are mid-range.
• Compressor failure on older equipment usually prompts a replacement comparison.

What this actually means

Every air conditioning failure lives in one of four layers: power, controls, air movement, or refrigeration. Working through them in order is faster than guessing, and it tells you immediately whether the next step is a five-minute homeowner fix or a service call.

When to Call a Professional

The breaker trips repeatedly.
The thermostat is dead after new batteries.
The outdoor unit will not start with power confirmed.
Everything runs but there is no cooling.
There is any burning smell or unusual electrical noise.

Preventing a Repeat

• Replace thermostat batteries annually.
• Flush the condensate drain each season so the float switch never trips.
• Have electrical components tested during maintenance.

Worth doing yourself

• Replace thermostat batteries every year.
• Reset a tripped breaker once and monitor.
• Flip the air handler service switch back on.
• Clear a visibly clogged condensate line to reset the float switch.
• Replace a dirty filter that may be part of a broader shutdown.

Do not waste money on

• Repeatedly resetting a breaker that keeps tripping.
• Opening the outdoor electrical compartment yourself.
• Assuming a thermostat replacement will fix a wiring short.

Frequently Asked Questions

Guidance this page draws on

U.S. Department of Energy — Guidance on routine homeowner maintenance and thermostat operation.
ACCA — Industry standards for residential HVAC system diagnostics.
CPSC — Electrical panel and breaker safety guidance.