Why Isn’t My AC Compressor Turning On? Diagnosing Common Causes and Solutions

Is your air conditioner not blowing cold air even though everything else seems to be running? If the AC fan is working but the air coming out isn’t cold, chances are there’s an issue with the AC compressor. The compressor is the heart of any air conditioning system, circulating refrigerant to remove heat from your home’s air. When it fails, cool air can’t be produced.

Fortunately, many common compressor problems are relatively easy to identify and address. Learning what causes AC compressors to malfunction can help you get your cooling system back up and running. In this article, we’ll discuss six of the most common reasons an air conditioner compressor won’t turn on and what you can do to fix them.

What Is an AC Compressor?

what is an ac compressor
What Is an AC Compressor?

Before diving into why your compressor might not be working, let’s quickly cover what exactly a compressor does. This important AC component serves several key functions:

  • Compresses refrigerant into a heat-absorbing liquid state
  • Moves refrigerant through the evaporator and condenser coils
  • Converts low-pressure refrigerant gas back into high-pressure liquid again

Powered by an electric motor, the compressor sends refrigerant on a continuous loop through your air conditioner’s sealed refrigeration system. As it changes between gas and liquid states, the refrigerant absorbs or releases heat to either cool or heat your home. Sophisticated sensors tell the compressor when to shut on and off to maintain your desired indoor temperature.

If the compressor isn’t running, refrigerant can’t circulate, eliminating the AC system’s ability to remove heat. Next we’ll look at six common issues that could be preventing your compressor from operating.

Power Problems

One of the most obvious causes of a compressor not turning on is a lack of power. Like all electrical devices, AC compressors need electricity to run. Power issues therefore should be your first focus when troubleshooting a non-working unit.

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Potential power problems include:

Tripped Circuit Breaker: Like most appliances, air conditioners require a dedicated 220-volt circuit protected by circuit breakers or fuses. If the circuit breaker trips, the compressor won’t receive power. Reset the breaker first before investigating other issues.

Blown Fuse: Similarly, a blown fuse along the compressor’s high-voltage circuit will cut off power. Check your AC unit’s external disconnect box for burnt-out fuses.

Faulty Wiring Connection: Over time, vibrations can loosen wiring connections to the compressor and condensing unit. Inspect wires for visible damage and ensure tight, secure connections.

Power Outage: Obviously, if your home or neighborhood loses power from the electric grid, your AC won’t work. You’ll need to wait for the outage to be resolved.

Improper Voltage: Air conditioning units require steady, consistent input voltage within a narrow tolerance. Too high or too low voltage can prevent compressors from activating. Contact an HVAC technician to test your unit’s electrical supply.

The simplest first step whenever an AC compressor fails to start is trying the reset button. Locate the small red button on the corner of the metal compressor box and press firmly. If faulty wiring is causing electrical overload shutoffs, a reset may clear the issue and allow operation.

If the reset button doesn’t work or power problems are confirmed, don’t keep attempting to run your air conditioner. This risks damaging the compressor. Instead, immediately call an electrician or HVAC technician based on where the issue seems to be.

Faulty Capacitor

After power problems are addressed, the next most common culprit for a seized compressor is a bad capacitor. This electrical component helps provide an initial power boost to start the compressor’s motor spinning. Over time, capacitors can fail or lose efficiency, preventing your AC system from powering on.

Warning signs of a broken capacitor include:

  • Compressor hums but doesn’t start
  • AC cooling intermittent and unreliable
  • High-pitched whining sound upon startup

Replacing a bad capacitor is a fairly simple, low-cost repair that HVAC techs can complete in under an hour. Capacitors are also designed for eventual failure, so it’s smart to proactively replace this part once your AC system reaches 5-8 years old. Preventative maintenance helps avoid breakdowns during peak cooling season.

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Failed Compressor Motor

The compressor motor converts incoming electrical energy into the mechanical power needed to operate the refrigerant pump. Motors can overheat and malfunction over time due to:

  • Years of wear and tear
  • Voltage irregularities
  • Lack of lubricating oil
  • Internal component breakdown

Warning signs your compressor motor has failed include:

  • Loud buzzing, scraping or grinding noises
  • Smoke or burning smell from the unit
  • Very hot external AC casing

Once an AC compressor motor burns out, the only solution is complete compressor replacement. Thankfully, swapping a new compressor unit in place is straightforward for HVAC technicians. And upgraded compressor technology can provide better efficiency and reliability than older models.

Faulty Thermostat

Before assuming the compressor itself has mechanically failed, be sure to rule out potential thermostat problems:

Incorrect Temperature Setting: If your thermostat is accidentally set higher than the current room temperature, the cooling system will remain off. Double check the thermostat setpoint.

Dead Batteries: Thermostats with low internal battery power can’t reliably communicate system demands to the compressor. Replace thermostat batteries annually before AC season.

Malfunctioning Sensor: Thermostats have a small internal temperature sensor to judge heating/cooling requirements. If this sensor fails, inaccurate temperature readings won’t properly trigger cooling cycles.

Faulty Wiring: Loose or damaged wires between your thermostat and AC unit can impact compressor operation. Inspect connections for flaws.

Any of these thermostat-related issues can prevent the compressor from receiving the electrical signals to start cooling. Professional HVAC service can quickly determine if the issue lies with your thermostat settings, power, sensor accuracy or wiring integrity.

Refrigerant Problems

At the heart of any air conditioning system is the refrigerant, an engineered chemical that absorbs and releases heat to provide cooling. So if your compressor won’t start, low refrigerant levels or refrigerant leaks are possible reasons. Some indicators include:

  • AC was working but slowly lost cooling capacity
  • Frost or ice buildup on exterior copper lines
  • Higher-than-normal compressor cycling on and off
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Over time, refrigerant can slowly seep through small leaks in fittings, seals or lines. Or major leaks caused by corrosion, wear or damage can rapidly depressurize the entire refrigerant circuit. The compressor may refuse to start as a fail safe. Running the compressor without adequate refrigerant flow can seriously damage the unit.

Refrigerant charging gauges and leak detectors allow HVAC pros to diagnose leaks needing repair, then properly recharge your AC system’s refrigerant level if low. With the refrigerant issue addressed, compressor operation can be restored.

Compressor Lockup

In rare cases, physical jamming inside the compressor can seize the internal pump and moving parts. This “locked rotor” event is usually caused by an electrical issue or lack of lubrication buildup associated with compressor failure in one of the previous categories. Warning signs include:

  • Compressor makes loud humming/buzzing but doesn’t start
  • Compressor motor feels abnormally hot to the touch
  • Burning plastic smells from compressor

Once an AC compressor completely jams and locks up internally, the only remedy is replacing the failed compressor. Thankfully full lockups are relatively uncommon compared to electrical or thermostat issues. And modern compressors feature improved lubrication and resilient internal components.

Getting Professional AC Help

getting professional ac help
Getting Professional AC Help

While many compressor operation problems come down to electrical overloads, low refrigerant levels or bad capacitors, only hands-on testing by a qualified technician can properly diagnose your AC system. Plus compressor repairs often require specialized equipment, professional experience and compliance with refrigerant recharging regulations.

So if you’ve confirmed the compressor is at fault for a malfunctioning, underperforming or non-operational AC system, calling a professional HVAC company is advised. Timely repairs can get your home’s vital cooling system humming again just in time for summer heat. Stay comfortable through the dog days by understanding common reasons for compressor shutdown and how to correctly address them.

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