How To Troubleshoot a 2-Stage Air Conditioner?

Most household air conditioners in the US use a single compressor, especially older models. Two-stage air conditioners, which are relatively new in the market, are complex and more challenging to maintain and repair than single-stage conditioners. 

To troubleshoot a 2-Stage air conditioner, check for the following: a tripped circuit breaker, a faulty capacitor, a faulty compressor, inadequate refrigerant, a damaged rectifier, or a clogged drain. Check the possible problems one by one until you pinpoint the cause. 

This article will discuss how to identify the most common problems in two-stage air conditioners and how to fix them. 

1. The Circuit Breaker May Be Tripped 

Check the fuses and circuit breakers since they are the most common reason your stage 2 air conditioner may malfunction. Problems caused by such components are often less serious than they seem.  

Always take safety precautions before handling electrical components like circuit breakers. Ensure the unit is off and has cooled down. 

A tripped circuit breaker is a sign that the air conditioner needed protection due to a surge in voltage, water leakage, or wiring problems. 

How To Fix?

Follow this process: 

  1. Turn the air conditioner off using the thermostat. 
  2. Go to the panel and turn the air conditioner’s circuit breaker switch to where it says “on.” 
  3. Wait for around half an hour while the air conditioner is off. It’s important that it stays off to reset the internal circuit breaker, which can’t happen if the thermostat signals for cooling. 
  4. Set the air conditioner back to cool. Call a professional if the circuit breaker immediately trips, as it signifies a complex wiring or electrical problem.

If the system usually works for a while, then the circuit breaker trips again, try changing the air filter. Sometimes, when the filter is clogged, it causes the motor to work harder, which can draw more electricity, causing a circuit break. 

Read: Top 5 Common Problems With Two-Stage Air Conditioners You Need To Know

2. The Thermostat May Be Damaged or Wrongly-Connected

When you turn on the air conditioner using the thermostat, it should start working. If it doesn’t, don’t automatically assume something is wrong with the unit. 

Sometimes, the thermostat may be damaged, or there may be a faulty connection between the thermostat and the air conditioning unit. Fortunately, it’s easy to check whether the air conditioning unit is damaged.

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How To Fix?

To troubleshoot an air conditioner that won’t start with the thermostat, locate the two low-voltage wires that connect the furnace to the compressor. On most models, these two wires are labeled Y1 and Y2. 

Connecting the Y2 wire to the system will force the furnace into the second stage, which will get your system to start cooling, even if the thermostat doesn’t work. 

Read: How To Fix Air Conditioner Leaking Water Inside?

3. The Capacitor May Be Faulty 

If your fan is running but blowing hot or room-temperature air and the unit outside isn’t running at all, you might have a faulty capacitor

Another indicator of a faulty capacitor is when both the indoor and outdoor fans are working, but the compressor isn’t cooling the air. 

How To Fix?

To replace a malfunctioning capacitor, follow these steps:

  1. Turn off the power. 
  2. Take out the old capacitor. Before you touch any wires, make sure the capacitor is discharged. 
  3. Get the corresponding wires in place and put your new capacitor in its place. 

Here’s a video that demonstrates how to replace the capacitor of a 2-stage AC: 

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4. The Refrigerant Levels May Be Low  

If your unit is running perfectly, but the house isn’t cooling properly and never reaches the set temperature on the thermostat, it may be a sign that your AC unit has been leaking refrigerant

This may also be the cause if your air conditioning unit often shuts down abruptly. When the levels of refrigerant drop, the system has to draw more power, which causes it to shut down.

How To Fix?

Unfortunately, replacing the refrigerant is tricky, and you’ll be better off involving a professional HVAC technician. The professional will need to locate leaks using a pressure monitoring device and repair or replace the coil or refrigerant lines. 

While you could simply refill the refrigerant, it will only leak out again, so it’s best to have a professional fix the problem for good. 

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

5. The Condensate Drain May Be Clogged 

If your AC’s condensate drain is clogged and you have a condensate overflow switch installed, the system will shut down. 

In such cases, you’ll notice a big puddle of water underneath your unit. While this isn’t a major problem, it can lead to more serious damage to your air conditioning unit, so get it fixed as soon as possible. 

How To Fix?

If your condensate drain is clogged, you’ll have to clean the drain line, which will automatically reset the switch and get the air conditioner running again.

If the drain line is glued, you might have to cut it open, clean it, then use a coupler to put it back on or glue it back together. You could also blow compressed air into the pipe to unclog it, which should get the unit running smoothly again.

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

6. The Amp Draw May Be Too High or Too Low

A high amp draw causes excessive power usage and may damage more sensitive equipment. On the other hand, while a low amp draw isn’t a major issue, it could lead to a system failure if left unchecked. 

Low voltage could also cause liquid to leak into the compressor, shortening its life span. 

How To Troubleshoot and Fix?

To check whether the amp draw is the issue, you’ll have to record your amp draws using the steps below:

  1. Find a suitable meter and set it to current. 
  2. Use the meter to record the amp draw for the first stage by ensuring that only the wire labeled Y1 is connected. 
  3. Record the amp draw for the second stage by connecting the wire labeled Y2. 
  4. If there’s an increase in amp draw between the first and second stages, the conditioner has successfully switched to the second stage, and it’s unlikely that you have an amp draw problem. 

You have an amp draw problem if your meter doesn’t show an increase between the first and second readings, meaning that the conditioner can’t get enough current to switch to the second stage. The next step is to get an HVAC professional or electrician to check the air conditioner and make any necessary repairs. 

If you don’t notice an increase in the two readings, have an HVAC professional or electrician check the system’s wiring and voltage. 

7. The Rectifier May Be Damaged 

Another possible reason your 2-Stage air conditioner isn’t working properly is a damaged rectifier delivering voltage to the compressor outside the recommended range. 

This will cause more problems in a one-stage air conditioner, but it still needs to be fixed if it occurs in your 2-stage conditioner. 

How To Fix?

To know whether a faulty rectifier is causing your problem, measure the voltage of the power that goes into the rectifier and compare it to the voltage of the rectifier’s output. 

The rectifier’s output voltage should be in the same range as the input voltage. But as the main role of a rectifier is to convert alternating current to direct current, the most important factor is whether there’s output voltage. 

If the meter doesn’t show any power from the rectifier, this component is likely the cause of the air conditioner’s malfunction, and you should replace it. 

8. The Compressor May be Faulty

As it is the soul of the air conditioning system, replacing a damaged compressor can cost almost as much as a new AC unit. If you notice vibrations in the condenser unit or clunking noises from your compressor, there may be a problem.  

If you’ve gone through the list of possible problems in this article and everything seems to be working perfectly yet your air conditioner unit still isn’t working the way it should, the compressor is most likely faulty.

How To Fix?

The only solution is to have a professional check the compressor. If they identify a minor issue, you can repair the compressor. Check whether you have a warranty for the AC unit, as compressor repairs can be costly. 

If the replacement cost is too high and your AC unit is old, consider replacing the entire unit. 


If your 2-stage air conditioner stops working, there’s usually no need to panic! Troubleshooting a 2-stage air conditioner is almost identical to troubleshooting a single-stage unit. The only difference is the compressor solenoid. 

Be careful while following the troubleshooting steps to avoid breaking down the air conditioning. Also, don’t attempt repairs if you don’t have HVAC repair experience. That’s what HVAC professionals are there for!