AC Not Working Troubleshooting (Troubleshooting Guide)

On this page

Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

Start by checking the thermostat settings and power supply. If the AC still doesn't work, inspect the circuit breaker and air filter. Replace the filter if dirty, and reset the breaker if tripped.

Confirm the symptom

No cool air coming from vents
AC doesn't turn on at all
Thermostat shows no response
Unusual noises when attempting to start

Tools and prep

Thermostat
Screwdriver
New air filter
Flashlight
Voltage tester
Gloves
Turn off power to AC unit at the circuit breaker before inspecting

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify the thermostat is set to 'Cool' and the temperature is lower than the room temperature.
Observe: Thermostat displays correct settings and responds to changes.
Means: Thermostat settings are correct.
Next: Check power supply
Check Power Supply
Do: Ensure the AC unit is receiving power by checking the circuit breaker and any wall switches.
Observe: Circuit breaker is on and not tripped.
Means: Power supply is intact.
Next: Inspect Air Filter
Inspect Air Filter
Do: Remove and examine the air filter for dirt or blockage.
Observe: Filter is dirty or clogged.
Means: Dirty filter may be restricting airflow.
Next: Replace or clean the filter
Replace or Clean the Filter
Do: Replace the air filter with a new one or clean it if reusable.
Observe: Airflow improves after replacing or cleaning the filter.
Means: Filter issue was resolved.
Next: Test AC Operation
Test AC Operation
Do: Turn the thermostat back to 'Cool' and check if the AC starts and blows cool air.
Observe: AC starts and blows cool air.
Means: Issue was likely a dirty filter or power problem.
Next: Problem resolved
Check for Error Codes
Do: Look for any error codes on the thermostat or indoor unit display.
Observe: Error code is displayed.
Means: Specific issue identified by the system.
Next: Refer to user manual for error code meaning
Inspect Outdoor Unit
Do: Check the outdoor unit for obstructions, ice buildup, or damage.
Observe: Unit is clear of obstructions and functioning without unusual noises.
Means: Outdoor unit is operational.
Next: Check refrigerant levels
Check Refrigerant Levels
Do: Look for signs of refrigerant leaks such as hissing sounds or ice on the coils.
Observe: No signs of refrigerant leaks.
Means: Refrigerant levels are likely adequate.
Next: Contact a professional if issues persist

Common causes

Cause	Likelihood	Difficulty	Part often needed
Dirty Air Filter Restricts airflow, causing the system to work harder or overheat.	Common	Easy	Air filter
Tripped Circuit Breaker Power interruption due to overload or short circuit.	Common	Easy	None
Thermostat Issues Incorrect settings or malfunctioning thermostat can prevent the AC from activating.	Sometimes	Easy–Medium	Thermostat
Refrigerant Leak Low refrigerant reduces cooling efficiency and can damage the compressor.	Sometimes	Hard	Refrigerant
Frozen Evaporator Coils Poor airflow or low refrigerant can cause coils to freeze, blocking airflow.	Sometimes	Medium	None
Faulty Capacitor A failed capacitor can prevent the compressor or fan from starting.	Rare	Medium	Capacitor
Worn Out Compressor An old or damaged compressor may fail to start or maintain pressure.	Rare	Hard	Compressor

Parts matching

Common parts needed for AC troubleshooting include:

Air filter – Replace if dirty or clogged
Thermostat – Check for proper settings or replace if faulty
Circuit breaker – Reset if tripped
Capacitor – Replace if faulty
Refrigerant – Recharge if there's a leak

Always match part specifications to your AC model for compatibility.

When to call a professional

AC still doesn't work after replacing the air filter and checking power
You hear unusual noises or smell burning from the unit
Error codes indicate a complex issue
Refrigerant leak is suspected or visible
Outdoor unit is frozen or has significant ice buildup
Thermostat replacement doesn't resolve the issue

FAQ

Why is my AC not turning on?

Check the thermostat settings, power supply, and circuit breaker. A tripped breaker or incorrect thermostat setting can prevent the AC from starting.

How often should I replace my air filter?

Replace the air filter every 1–3 months, depending on usage and filter type, to maintain efficient airflow.

Can a dirty air filter cause the AC to stop working?

Yes, a dirty filter restricts airflow, which can cause the system to overheat or shut down to prevent damage.

What should I do if my AC is making loud noises?

Turn off the unit and check for obstructions. If the noise persists, it may indicate a mechanical issue that requires professional attention.

How can I tell if my AC has a refrigerant leak?

Signs include reduced cooling, hissing sounds, or ice buildup on the coils. Refrigerant leaks require a professional to repair and recharge the system.