How To Fix It

Top 5 Common Problems with Two-Stage Air Conditioners You Need To Know

As their name implies, two-stage air conditioners have two modes of operation depending on surrounding weather conditions. Like other air conditioners, these appliances can encounter issues now and then. 

Common 2-stage air conditioner problems include bad, old, or damaged capacitors, damaged air filters, clogged condensate drains, and bad condenser fan motors. Solutions include changing the air filters, motors, and capacitors and unclogging or replacing the condensate drains.

This article will help you troubleshoot five common issues with 2-stage air conditioners. You can do some of these on your own, while others may require the assistance of a certified HVAC professional. 

1. Faulty Capacitor

The capacitor is a small device inside your AC unit’s cabinet and is usually near a motor. Essentially, capacitors store the charge needed to jumpstart and run your compressor motor as it begins the cooling process.

Your two-stage air conditioner likely has dual-run capacitors, meaning they support two motors — the fan motor and compressor motor. On average, dual-run capacitors can last 20 years before they need replacement.

Capacitors can fail because:

  • They get old.
  • They were exposed to too much heat.

To fix a damaged capacitor, you must take your AC unit apart with special tools. As this involves tricky electrical work, I recommend contacting a certified electrician to help you out unless you have solid DIY skills. 

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Read: How To Fix Air Conditioner Leaking Water Inside?

How To Fix?

First, you need to confirm that your AC’s capacitor is damaged. The signs of a faulty capacitor are as follows.

  • The fan in your indoor unit is running while the fan in the outside AC unit isn’t. 
  • The indoor and outdoor fans are working, but your compressor isn’t.
  • There’s an irritating buzzing noise in your unit. Normally, AC units make a low-pitched rumble while running. If your AC unit makes any other kind of noise, there’s likely something wrong with it.
  • The AC unit won’t turn on right away or at all.
  • The AC turns off on its own.

To determine if the AC capacitor is the issue, you need to test it with a multimeter. You can purchase something like the BTMETER BT-7200APP TRMS 6000 Counts Clamp Multimeter (available on Amazon.com). You can connect it to your phone and get an accurate reading of your AC’s capacitor’s voltage, current, and amperage. 

If all signs point to a faulty capacitor, you’ll need to replace it.

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2. Damaged or Clogged Air Filters

If your AC isn’t blowing cool air, your air filter may be clogged, disrupting the flow of air from the unit. As a general rule, you should clean your AC’s filters every month or quarter. Otherwise, the AC will work harder to cool you, and your energy bills will skyrocket. 

Read: Do You Have To Vent A Portable Air Conditioner In Heat Mode?

How To Fix?

Assess your AC filter for any signs of damage, such as visible cracks or unexplained dents on the filter surface. If the air filter is damaged, you must get a new one. 

When you’re choosing a replacement filter, you need to account for its MERV rating (Minimum Efficiency Reporting Value). The MERV measures how many air particles of a certain size your filter can capture. For example, a MERV 13 home AC filter should be enough to supply high-quality indoor air to your home.

3. Clogged Condensate Drain

The condensate is the PVC pipe that runs from your central AC to the outside. It carries the water from the condensed moisture the evaporator collects and dumps it near the outdoor unit. A clogged condensate drain must be cleared out since it can lead to problems such as water freezing inside the evaporator coil.

Signs of a clogged AC condensate drain include:

  • Your indoor AC unit is dripping water
  • The AC is not turning on
  • Higher than normal energy bills
  • Mold and mildew damage caused by damp conditions near your inside unit

How To Fix?

If the condensate drain is at fault, you can treat the pipe with a soap solution as follows.

  1. Remove the drain pipe cap.
  2. Check if there’s anything blocking the pipe.
  3. Mix warm water with dish soap.
  4. Flush the water and soap solution down the pipe.

You may see some people recommend that you clean your drain with vinegar or baking soda. However, vinegar may cause the pipe to smell funky, while baking soda can leave a mess if not handled properly. Unless your condensate drain is seriously clogged, I recommend a gentle cleaning method first. 

Read: Why Is My Air Conditioner Not Dripping/Draining Water?

4. Bad Condenser Fan Motor

When you turn on your AC, you should hear the fan motor move or at least feel a soft breeze from the fan action. If you don’t hear the fan rotate or if it runs too slowly, you may need to check it for damage. 

Other signs of a damaged fan motor are as follows:

  • Everything but the condenser fan works.
  • The fans won’t turn on when the unit is running.
  • Smoke is coming from your unit, which could indicate an electrical fault.

Depending on how far the fan blades are, you can use a long, straight stick to nudge them into a soft spin. Fan motors should rotate easily when the system is on. If they don’t, dirt may have accumulated in the motors. 

In the worst-case scenario, you may have to replace your fan motor altogether. 

How To Fix?

Because of the nature of the fan motor, the best course of action is to ask for professional help. The electrician can check your AC and determine if the fan or its motor is the issue. Either way, prepare to set aside a bit of cash to pay for the replacement part if your AC or the type of damage isn’t covered by the warranty.

5. Faulty Thermostat

Any issues with the thermostat will cause issues with your two-stage AC. For example, if the thermostat settings are wrong, the system won’t switch stages properly.

The following signs could indicate a problem with the thermostat:

  • Temperature doesn’t adjust to the settings.
  • The thermostat doesn’t have power.
  • The system doesn’t turn off.

You also need to check the thermostat wiring. Any issues with the same can also lead to issues with the AC.

Read: Can An Attic Be Used To Vent A Portable Air Conditioner?

How to Fix?

  1. Check the thermostat’s wiring in your manual. Make sure all the wires are properly connected. If they’re not, rewire them according to your manual. Check for signs of damage as well.
  2. Locate the differential temperature setting for heating and cooling in your thermostat. Differential temperature refers to the temperature difference between the inside and outside temperatures.
  3. Drop the differential temperature by a degree and test if the second stage kicks in. Call in a professional if that doesn’t work.

Other issues, such as faulty condenser control boards, may cause issues similar to a faulty thermostat. I should note that if the condenser control board is the issue, you’ll have no choice but to replace it. For that reason, I recommend getting in touch with an AC repairman to get the unit fixed. 


Like other AC units, two-stage air conditioners require regular maintenance. Otherwise, you’ll find yourself saddled with costly repairs that could’ve been avoided had you taken care of your AC unit properly. If you experience any of the above issues and don’t know how to fix them, it’s best to ask a certified HVAC technician for help.