What is the fastest thing to check?

The thermostat fan setting. Set it to AUTO — a fan stuck on ON delivers room-temperature air between cooling cycles.

Can low refrigerant fix itself?

No. Refrigerant is not consumed, so low charge always means a leak that needs to be found and repaired.

Is this an emergency?

Usually not, but in extreme heat or with vulnerable occupants at home, treat loss of cooling as urgent and seek same-day service.

Why Is My AC Not Cold?

Causes, safe checks and when to call a professional

Expert Answer

The short version: your AC is not cold because either it is not actually running the cooling cycle, or it is running but cannot move heat. A two-minute test tells you which. Set the thermostat to COOL and the fan to AUTO, wait five minutes, then go outside. If the outdoor unit is running and the vents still are not cold, it is a refrigeration or airflow problem needing service. If the outdoor unit is not running, it is an electrical or control problem — also needing service, but a different repair.

Check these 3 things first

1Measure vent temperature with a thermometer rather than relying on feel.
2Check indoor humidity, since high humidity makes air feel warmer than its actual temperature.
3Confirm the thermostat setpoint is meaningfully below the current room reading.

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. Thermostat mode or fan setting
COOL mode with AUTO fan is required. Anything else produces uncooled air.
2. Filter restriction
A loaded filter is the most common homeowner-fixable cause of weak, uncold air.
3. Compressor not running
No compressor, no cold. Electrical faults and lockouts are the usual reasons.
4. Low refrigerant charge
The system can only remove heat if it has the right amount of refrigerant circulating.
5. Dirty coils
Fouled indoor or outdoor coils cut heat transfer on either side of the cycle.

Safe Homeowner Checks

Thermostat: COOL mode, fan AUTO, setpoint below room temperature.
Filter: replace if dirty.
Outdoor unit: confirm it runs when the thermostat calls.
Registers: confirm they are open and unobstructed.
Breaker: confirm both indoor and outdoor breakers are on.

Stop and get help if you notice

• Ice on the copper lines.
• Repeated breaker trips.
• Burning smell during operation.

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.

Perceived versus measured temperature check
About 5 minutesTools: Basic thermometer
Tests for: Thermostat mode or fan setting
1. Hold a thermometer at a supply register for one minute.
2. Hold the same thermometer at a return grille for one minute.
3. Subtract the two readings.
Normal result
Supply air reads roughly 16-22°F colder than return air.
Problem result
The difference is much smaller than that, even though the air feels somewhat cool.
What it means: A small measured split means the system is not cooling well even if the air feels acceptable subjectively.
Filter check
About 2 minutesTools: None
Tests for: Filter restriction
1. Remove and inspect the filter.
2. Replace it if visibly loaded with dust.
Normal result
Filter is clean and allows light through.
Problem result
Filter is dark and clogged.
What it means: A restricted filter reduces the temperature split even when the compressor is working normally.
Outdoor unit run confirmation
About 3 minutesTools: None
Tests for: Compressor not running
1. Go outside during a cooling call.
2. Confirm the fan spins and a steady hum is present.
Normal result
Unit runs continuously without cycling off quickly.
Problem result
Unit is off, or clicks on and off within seconds.
What it means: The compressor is not running long enough or at all, so no real cooling is occurring.
Comfort versus humidity distinction
About 5 minutesTools: None (optional indoor hygrometer)
Tests for: Low refrigerant charge
1. Note if the house feels sticky or clammy rather than simply warm.
2. Check an indoor humidity reading if you have one.
3. Compare with how the house has felt in past cooling seasons.
Normal result
Indoor humidity sits in a comfortable range and the house has felt this way before.
Problem result
Humidity feels notably higher than usual alongside weak cooling.
What it means: Low refrigerant charge reduces both temperature drop and moisture removal, making the space feel warm and humid together.

Want a pro to confirm it?

We map out the causes for free. If you'd like a licensed local pro to take a look, drop your email and ZIP and we'll connect you with top-rated experts — no pressure, no obligation.

What This Typically Costs

Filter replacement
DIY
$15-$30
Professional
$40-$80
Typical time
10 minutes
Capacitor/contactor repair
DIY
Not a DIY repair
Professional
$150-$400
Typical time
1 hour
Refrigerant leak diagnosis and repair
DIY
Not a DIY repair
Professional
$300-$1,800+
Typical time
2-6 hours
Coil cleaning
DIY
$0 (hose rinse)
Professional
$100-$250
Typical time
30-60 minutes

Typical repair costs and time
Problem	DIY cost	Professional cost	Typical time
Filter replacement	$15-$30	$40-$80	10 minutes
Capacitor/contactor repair	Not a DIY repair	$150-$400	1 hour
Refrigerant leak diagnosis and repair	Not a DIY repair	$300-$1,800+	2-6 hours
Coil cleaning	$0 (hose rinse)	$100-$250	30-60 minutes

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 outdoor unit run when you lower the setpoint?
Yes: Refrigeration or airflow problem — schedule diagnosis.
No: Electrical or control problem — check breakers, then schedule service.
Is the filter clean?
Yes: Move on to the refrigeration side.
No: Replace it and retest for an hour.

What the technician will check

Temperature split measurement.
Refrigerant charge verification.
Compressor electrical testing.
Coil inspection.

What this actually means

"Not cold" describes temperature, not airflow. Any time the system runs without producing cold, the refrigeration cycle is the suspect: the compressor, the charge, or the coils. Settings and filters are the two homeowner-fixable exceptions worth ruling out first.

When to Call a Professional

Settings and filter are ruled out and the air is still not cold.
The outdoor unit does not start.
Ice appears anywhere on the system.

Preventing a Repeat

• Change filters regularly.
• Book annual maintenance.
• Keep the outdoor unit clean.

Worth doing yourself

• Measure vent and return temperatures with a simple thermometer instead of guessing by feel.
• Replace the filter.
• Track indoor humidity to separate a comfort problem from a temperature problem.
• Confirm the setpoint is at least 3°F below room temperature before judging performance.

Frequently Asked Questions

Guidance this page draws on

ASHRAE — Comfort perception and humidity's role in perceived temperature.
ENERGY STAR — Practical vent temperature and filter guidance.