Compressor Not Running On AC Unit: Causes, Diagnosis, And Repair Guide For Homeowners

When the compressor is out on an AC unit, your cooling system can’t do its job properly—leading to discomfort, higher energy bills, and potential system damage. This guide explains why your air conditioner’s compressor may not be running, how to troubleshoot the issue, and what steps to take for repair. Homeowners will find up-to-date information, actionable tips, and answers to frequently asked questions.

Issue	Typical Causes	Action Steps
Compressor Won’t Start	Power issue, thermostat problem, capacitor failure, contactor defect, overload	Check power supply, thermostat, inspect capacitors and contactor
Clicks But Does Not Run	Failed start relay, seized compressor, electrical fault	Test relay, check compressor windings, consult technician
Runs Briefly and Shuts Down	Overheating, low refrigerant, pressure switch open	Clear coils, check refrigerant, examine pressure safety

How an AC Compressor Works

The compressor is the heart of your air conditioning system. It pumps refrigerant through the AC’s coils, enabling heat exchange and cooling. When the compressor isn’t working, the AC can’t remove heat from your home, usually resulting in warm air blowing from the vents even when the system is running.

Typically found outdoors in the condensing unit, compressors switch on and off as dictated by your thermostat settings and system demand.

Common Signs The AC Compressor Is Out

• AC Runs But Blows Warm Air: The fan works, but no cooling occurs.
• Outdoor Unit Makes Clicking or Humming: You hear noises, but the compressor does not start.
• Unit Won’t Start: No response from the outdoor unit when the AC is called for.
• Frequent Tripping of Circuit Breaker: The compressor draws too much current or has an internal short.
• Visual or Audible Signs: Burned wires, smoking, or unusual odors from the outdoor unit.

Top Causes Why a Compressor Is Not Running

Electrical Problems

Tripped Breaker or Blown Fuse: If a breaker trips continuously, it may indicate a short circuit or overload in the compressor circuit.

Faulty Contactor or Relay: These components supply and cut power to the compressor. When malfunctioning, the compressor may not get the voltage it needs to start.

Bad Capacitor

The start capacitor supplies a jolt of electricity that helps the compressor start up. If the capacitor is bad, the compressor may hum, click, or do nothing at all.

Thermostat Issues

A malfunctioning thermostat can prevent the AC from signaling the compressor to operate. Double-check your thermostat settings and replace batteries if necessary.

Compressor Motor Failure

Internal mechanical failures, such as a seized motor or shorted windings, require compressor replacement. These issues often result in repeated breaker trips or no sound from the compressor at all.

Low Refrigerant Levels

Low refrigerant can cause pressure switches to open, preventing the compressor from running to protect itself from damage. This situation is often caused by leaks in the system.

Overload or High Temperature Shutdown

Dirty condenser coils, blocked airflow, or high outdoor temperatures can cause the compressor to overheat and shut off for protection. After cooling down, it may briefly attempt to restart before shutting off again.

Wiring and Connection Problems

Loose, damaged, or corroded wires can cut power to the compressor. Visual inspection of the control board and outdoor unit wiring can sometimes reveal obvious problems.

Step-By-Step Compressor Diagnosis

1. Verify Power to the Outdoor Unit

Check the circuit breaker labeled for your AC system in your electrical panel. Ensure it’s switched on, and reset it if needed.

Test the disconnect box near the outdoor unit; verify the fuses are intact.

2. Inspect the Thermostat

Set the thermostat to “cool” and push the temperature below room temperature. Listen for the click that signals the system to start.

If unresponsive, replace thermostat batteries or check wiring for damage.

3. Examine the Capacitor and Contactor

With power off, carefully remove the panel cover of your outdoor AC to inspect the capacitor (usually a cylindrical or oval metal can). If it’s bulging or leaking, it needs to be replaced.

The contactor looks like a relay or switch; pitted or burnt contacts indicate failure.

4. Check Compressor Overload and Pressure Switches

Use a multimeter to test for continuity across the compressor’s overload protector and pressure switches. If open (no continuity), one may have tripped due to overheating or other issues.

5. Test Compressor Windings

With a multimeter, check the terminals labeled “C,” “R,” and “S” on the compressor for proper resistance values. Incorrect readings can indicate internal compressor damage.

6. Inspect For Refrigerant Issues

Low refrigerant usually results in ice around the evaporator coil or hissing sounds near joints. An EPA-certified technician is required to properly check, charge, or repair refrigerant lines.

Homeowner Checklist Before Calling a Professional

• Check Thermostat Settings: Confirm it’s set to cool and that batteries work.
• Reset the Circuit Breaker: Even if it looks on, flip it off and back on.
• Replace or Inspect Air Filter: Dirty filters can restrict airflow, leading to compressor shutdowns.
• Clear Debris Around Outdoor Unit: Ensure at least two feet of clearance for airflow.
• Listen for Noises: Humming or clicking noises can help diagnose the problem.

If these steps don’t resolve the issue, proceed with professional AC repair services for further diagnosis.

When to Repair vs. Replace a Compressor

Compressor replacement is expensive and is often the most costly single component swap in standard residential AC units. Deciding whether to repair or replace your AC depends on several factors:

• Age of Unit: If your system is over 10 years old, replacement may be more cost-effective.
• Refrigerant Type: Older systems using R-22 refrigerant are increasingly obsolete. Replacement is often recommended.
• Warranty Status: Many compressors have 5-10 year manufacturer warranties; check your documentation.
• Overall Condition: If other issues exist (leaks, corroded coils), total system replacement may offer better long-term value.

Situation	Recommended Action
Compressor in Warranty, Rest of System Sound	Replace Compressor Only
Unit Out of Warranty, Frequent Breakdowns	Replace Entire AC Unit
Minor Electrical Issue Only	Repair Component & Keep Compressor

Costs Associated with Compressor Repair and Replacement

Compressor repair or replacement costs can range from $900 to $2800, depending on your AC type, size, and location. If the issue is electrical (like replacing a capacitor or contactor), repair costs are usually between $100 and $400.

Full system replacements can cost from $4,500 to $12,000 depending on system size and efficiency. Always compare quotes and check for local rebates or incentives for high-efficiency upgrades.

Common Questions About Compressors Not Running

Can I Replace a Capacitor Myself?

Capacitors can be replaced by homeowners with basic electrical skills. Always turn off power at the breaker and use insulated tools. Discharge the old capacitor with a screwdriver (using the insulated handle) before removal.

What Causes A Compressor To Overheat?

Restricted airflow, dirty coils, high outdoor temperatures, or low refrigerant are typical culprits of compressor overheating. Regular maintenance, such as cleaning coils and replacing air filters, can prevent many causes.

How Can I Prevent Compressor Failure?

• Yearly professional HVAC maintenance checks
• Replace or clean filters monthly during peak use
• Keep outdoor unit clear of debris, grass, and shrubs
• Promptly repair leaks to prevent low pressure
• Ensure proper electrical grounding and surge protection

How Professionals Diagnose and Repair AC Compressor Problems

Certified HVAC technicians use specialized tools (amp meters, refrigerant gauges, and diagnostic computers) to pinpoint compressor issues. The repair process often involves:

• Measuring voltage and current at the compressor terminals
• Testing start and run capacitors for capacitance and leakage
• Checking for refrigerant leaks and pressure imbalances
• Examining circuit boards for burn marks or failed solder joints
• If needed, isolating and removing the compressor for bench testing or replacement
• Testing full-system operation following repairs

Preventive Maintenance Tips For Avoiding Compressor Problems

• Schedule professional tune-ups every spring to ensure all electrical components work properly.
• Check and clean indoor and outdoor coils regularly for maximum heat exchange efficiency.
• Monitor refrigerant levels and pressures at least once a year, especially before summer.
• Keep a maintenance log so recurring issues are documented.
• Install a surge protector on your outdoor AC disconnect to protect against lightning and power surges.

What To Expect From an AC Repair Service Visit

When you call a professional, expect a detailed inspection of electrical components, refrigerant pressures, and compressor function. The technician will present repair or replacement options, cost estimates, and guidance on the best steps for your system’s age and condition.

Tips For Finding A Reputable AC Repair Technician

• Look for NATE-certified or EPA-licensed HVAC technicians.
• Read reviews and verify licensing/bonding in your state.
• Ask about warranty policy and satisfaction guarantee for repairs.
• Compare quotes, but beware of extremely low bids (they may involve subpar parts or workmanship).

Long-Term Solutions For Compressor Problems

In some cases, upgrading to a modern, high-efficiency air conditioner can save on long-term energy and repair costs. Modern systems have improved reliability, better warranties, and advanced diagnostics to alert you to small problems before major failures occur.

Key Takeaways For Homeowners With Compressor Out On AC Unit

• Early diagnosis and repair prevent costly damage or replacement.
• Routine maintenance is your best defense against compressor failure.
• If your AC is near the end of its life, upgrading may be the smartest investment.
• Work with certified, trustworthy professionals for major repairs.