AC Not Working: 9 Steps How To Fix AC (By HVAC Technician)

When AC stops working, we want to fix it immediately. That’s only normal. AC not working can mean hell on Earth. It’s thus not unusual that a lot of people come to us asking how to fix AC. Here are just some questions regarding AC not working:

“Why is my AC running but not cooling?” (Possible Refrigerant Leak)

“What causes an air conditioner to stop cooling?”

“How do I get my AC to work again?”

“What to do if AC stops working?”

“What causes an AC compressor to stop working?”

These questions are very common. So much so that an HVAC technician has shared (on Reddit’s r/HVAC) a simple-to-understand foolproof plan of how to fix an air conditioner when it stops working.

He used this diagram to teach HVAC apprentices what to check for when an AC stops working.

We can all learn from it, right? DIY AC fixing can save you up to $1000.

Probably the best commentary by another now HVAC technician was that “This AC not working checklist has taught me more in 5 minutes than my professors taught me in 2 months”.

The diagram is designed for HVAC apprentices but everybody struggling with failing AC unit can have a look and try to fix the air conditioner on his/her own. Here’s the step-by-step ‘AC not working, how should I fix it?’ diagram:

9 Steps How To Fix AC (Like An HVAC Technician Would)

Disclosure: At first, the diagram might seem a bit complex and HVAC vocabulary a bit weird for an Average Joe. Hopefully, we’ll navigate through the HVAC lingo successfully.

Below the diagram, we will go step by step and explain what you should do to fix your own AC:

step by step of how to fix a diagram
Source: Reddit’s r/HVAC

Let’s start with the most obvious thing an HVAC technician has to do to start fixing an air conditioner:

1. Get A Meter, Tool Bag, And Gauges

One of the biggest mistakes, when we try to fix an AC that stopped working, is not to grab the toolbox. Looking at filters, checking evaporation coils, measuring the airflow output is impossible without some tools and gauges.

You probably don’t have many HVAC tools lying in your garage. It would make sense that, over the years, you add them one by one and be ready whenever an AC stops working, the furnace starts making weird noises, or you have other HVAC-related problems.

The basic toolbox of essential HVAC tools should include screwdrivers, pliers, hammer, flashlight, wrenches, cutters, drills, gauges (refrigerator gauge is a must-have), cords, staple gun.

A professional HVAC technician – the one who actually comes and fix an air conditioner that stopped working – should also include the following tools that are specifically designed for fixing ACs:

  • Core removal. Used for removing and replacing the core.
  • Coil fin straightener. Used for clearing air conditioner condenser coil debris, clogging, or wire twists. By clearing all those things, you can influence the air conditioner’s heat exchange and airflow.
  • Thermometers. When the air conditioner stops cooling, we have to measure the actual temperature of the output air. We will check how well we fixed an air conditioner by measuring how much the output air’s temperature dropped.
    thermometer in fahrenheit
    A simple thermometer is one of the most useful HVAC tools to detect air conditioner problems.
  • Multimeter. It’s a safety measure; when close to live wires, measure the voltage and amps, to make sure there is no electricity in the wire you’re about to touch.
  • Caulking gun. If the AC stopped cooling due to refrigerant leakage or a hole in the duct system, you can use the caulking gun to seal the air ducts.
  • HVAC pump. Used for removing moisture from the air conditioner lines.
  • Nitrogen regulator. In an air conditioner, the refrigerant is under high pressure (especially in the discharge line and condenser coil). A nitrogen regulator is a pressure control valve that reduces the pressure, making the AC easier to handle.
  • Leak detector. AC not working can be a consequence of refrigerant leakage. The leak detector is used to detect areas with above-average moisture; there is where the leakage is likely to be.
  • Psychrometer. Basic and important meter to measure the airflow (in CFM) and relative humidity (in %).
  • Thermal imagining tools. These are advanced but very useful tools to see the temperature of the whole air conditioner airflow. You can detect if your AC stopped cooling due to refrigerant leakage or a hole in the outer shell with ease.
  • Flushing solvent. Used to flush out obstructions within the line.

You don’t need all of these HVAC tools for a basic air conditioner checkup and smaller fixes. ServiceTitan has a good article about all the HVAC tools if you’re interested to read more.

Digital thermometer, psychrometer, and multimeter are the best basic trio everybody trying to fix an air conditioner should have.

2. Is The Condenser Coil Clean?

The condenser coil is located in the outdoor unit. Its main job is to release the excess heat; without that, the air conditioner cannot cool properly.

If you ask any HVAC technician how to fix an air conditioner, he or she will probably tell you to first check if the condenser coil is clean.

The condenser coil is exposed to all kinds of weather, dirt, even mold. If the coils are dirty, the heat won’t be able to dissipate properly, and the effect will be that the air conditioner will blow less cool air.

Here’s how can use a simple thermometer to check for this:

Measure the temperature of the air coming from the air handler. If the temperature is higher than a year ago, let’s say, it’s likely that AC is not working properly due to all the dirt gathered on the outdoor coils.

These could result in:

  • Overheated compressor.
  • Impeded airflow throughout the air conditioner.
  • Lower refrigerant levels.
  • Higher utility bills. The SEER rating of mini-split air conditioners might decrease drastically.
  • Even ice forming on the evaporator coils (indoor unit).

Checking and cleaning condenser coils is not a piece of cake:

First check the coils in the outdoor unit for dirt, leaves, soot, and inspect them for mold growth. If you see that the coils are dirty, clean them. Here’s what you need for cooling outdoor AC coils:

  • Safety first. Gloves, eyewear, knowing coils could be super hot.
  • Fin comb.
  • Coil brush.
  • Foaming coil cleaner.
  • Hose.

The coils are made from metal. You have to use a fin comb and/or coil brush to properly clean them. It can take quite a while but if that truly is the problem that’s making your air conditioner not cool as it was supposed to, it is actually the only thing you have to do.

Fair warning: Condenser coils can be quite hot, especially if the air conditioner is not functioning properly. A quick light touch is recommended to check if the temperature of the coils is cool enough for you to clean them safely.

After you clean the air conditioner condenser coils, check if the air conditioner is again cooling properly. You can ‘feel’ the difference in the temperature or you can use the digital thermometer to measure the temperature of output air before and after the cleaning.

3. Verify Condenser Fan Motor And/Or Capacitor

The fan is an essential part of every AC. Its function is to create airflow throughout the unit. Here’s how the line of operations goes:

The fan is powered by a motor. The motor is powered by the capacitor.

If your AC is performing poorly, or if it stopped working altogether, the problem might either be the condenser fan motor, or the capacitor. You have to check both of them.

Problems with a faulty fan are quite common. Check motor and capacitor.

To see if this really is the root of the problem why your AC stopped working, check for the signs of AC working improperly, such as:

  • Fan won’t start. Even when you turn the AC on, the propellers won’t start spinning.
  • Fan won’t stop. When you turn the air conditioner off, the blades are still spinning.
  • Slowly rotating blades. Fan is functioning but not at the required or expected speed. Psychrometer will show lower than specified airflow.
  • Fan is loud. Rattling noises within the condenser unit (outdoor unit) are usually associated with fan problems.

If you notice any of these signs in your malfunctioning air conditioner, you might have just successfully diagnosed the problem. Here’s how to fix an air conditioner if your fan has issues:

First, check the motor in the outside unit. You’ll need to unscrew the side panel to get access to the motor. Condenser fan motor malfunctions quite regularly. Here’s how to check and fix the motor:

  1. Breaker tripped. If you have a tripped breaker, the motor won’t work. Every breaker has 3 settings: “On”, “Off”, “Neutral”. If you see any breaker in the “Off” settings, move it to the “Neutral” setting before switching it to the “On” setting.
  2. Clogged air filters. It seems like a small thing but saturated air filters can limit the airflow, and cause a chain reaction that impedes the ability of the motor to function properly. Clean them, if possible, or replace them.
  3. Remove blade obstructions. Fan blades in the outdoor unit can collect falling leaves, small branches, or even dead birds. Removing the obstructions and allowing for the fan to spin unimpeded can fix an air conditioner.
  4. Manually start the fan. If you turn the AC on, and the blades aren’t spinning, try to kickstart them (be careful, of course). If they start spinning normally, the motor is not the problem, the capacitor is where the issue is most likely to be.

Secondly, check the capacitor. Capacitor powers the motor (also kickstarts it). It’s hidden in the outdoor condenser unit, and looks like a car battery. It’s connected to the rest of the system via wires.

If your AC stopped working and the problem is the capacitor, the top of the capacitor will be visibly swollen. If the top is flat, the capacitor is not the cause an AC stopped cooling.

The malfunctioning capacitor cannot be fixed; you’ll have to buy a replacement capacitor.

After you have checked everything, or even replaced the capacitor, you should test the AC fan motor. In HVAC lingo, this is called a ‘continuity test’. It includes checking for power, windings, and inspecting the capacitor. You can read more about how to do the continuity test here.

4. Are The Filters Clean?

Cleaning and regularly replacing AC filters is a part of basic maintenance. Dirty filters can impede the airflow, causing a chain reaction that can bring an AC to a stop.

Here is a range of problems dirty AC filters can cause:

  • Short-Cycling. During the day, AC cycles “On” and “Off”. Dirty filters can make these cycles more frequent, louder, and unnecessarily waists energy while producing higher temperature changes than needed.
  • Filters actually freezing. Filters should let the cold air through. If the air conditioner filters are clogged up, the full force of that cool air is continuously hitting the filters. This may lead to filters freezing and causing further malfunctions throughout the air conditioner via impeded airflow.
  • Uneven cooling. If parts of the filter are more clogged than others, the airflow will not be uniform. That will cause uneven cooling and higher electricity bill.
  • Allergens building up in the filters. If the filters are not regularly maintained, allergens can accumulate in the filters and are carried to your lungs, skin, and so on via the air conditioner airflow.

How to fix an air conditioner with dirty filters is quite straightforward. Clean them, if possible, or replace them.

5. Is The Evaporation Coil Clean?

The evaporator coil in the part of the indoor unit (air handler). This is the very coil that absorbs the heat from your home and makes the whole air conditioner capable of cooling. If anything would be wrong with the evaporator coil, the air conditioner stops cooling.

Evaporation coils tend to be cleaner than outdoor-located condenser coils. That doesn’t mean they can’t get dirty. If you don’t use an air purifier, indoor air pollutants like small particulates, dust, pollen, and mold can attach to the coils.

Here are the air conditioner issues associated with unclean evaporation coils:

  • Impeded heat transfer (immediate consequence).
  • AC cools less or even stops cooling.
  • Lower energy-efficiency of the air conditioner.
  • Increased wear-and-tear on the system.

To check if the coil is clean, open the air handler. Evaporator coils are made out of copper and surrounded by a series of aluminum fins that facilitate heat transfer (similarly to condenser coils).

Here is how you can find the evaporator coil, and clean it.

Cleaning the coils might be how you fix your air conditioner. However, cleaning the evaporator coils does require quite a lot of work.

You can use the same tools you use to clean the condenser coils, no-rinse evaporator coil cleaners, even paintbrushes. Make sure to remove all the dirt for the best energy-efficiency results.

6. Check Blower Motor Belt (It Should Be Taut, Not Loose)

An issue with the blower motor belt is a signal for alarm. It usually requires hiring professional assistance.

Older air conditioners still use blower fan belts. Modern AC units use direct motors to power the fan.

Blower motor belt is quite susceptible to damages, and the natural wear-and-tear that comes with aging air conditioners.

If you hear a loud clanging noise or clicking noise in the air conditioner, the usual suspect is the blower motor belt.

sketch of a non-working ac and hints how to fix it
Air conditioner is quite a complex system. The motor belt has to work for 10-25 years and could become loose rather quickly.

Fixing a blower motor belt is hard to DIY. The best most of us can do is to diagnose the problem and see if we actually need to call an air conditioner repair guy.

Here’s how to check if AC is not working due to a faulty motor belt:

  • Check the belt’s tension.
  • If the belt is taut, the belt is not the problem. You have to check other ways how to fix an air conditioner.
  • If the belt is loose, you’ve correctly diagnosed the issue with your AC. Call an HVAC technician to come and replace the faulty belt.

7. Check Blower Motor And Capacitor

A faulty capacitor is one of the 5 most common reasons why an air conditioner is not working.

A capacitor is an essential part of the AC; it stores energy via the use of an electrostatic field. Capacitor powers all the motors inside the air conditioner; compressor motor, blower motor, and outdoor fan motor.

All of these motors have a separate capacitor that powers them. Example: “Start Capacitor” starts up the AC, and “Run Capacitor” keeps the air conditioner running. It’s important to understand these are two separate capacitors; one or the other can be faulty if the AC is not running properly.

heat exchange air conditioner capacitor facilitates
A working compressor is a basic requirement for seamless heat exchange.

“Hard starting”, for example, is a consequence of a faulty “Run Capacitor”. The capacitor’s function is to provide the stable 120V or 220V voltage to the motors.

Check both all the capacitors to see if they are running smoothly. Also, check with the voltmeter how many volts go into powering each of the motors.

You will find the compressor in the outdoor unit; unscrew the top and find the battery-like element.

Example: “Run Capacitor” should create 120V voltage to power up the starter motor. If you measure 40V with a voltmeter, you know that that is the faulty capacitor.

How to fix an air conditioner with a faulty capacitor?

Simple. Just replace the capacitor.

8. Fixed Metering Device Or TXV?

Here is this HVAC lingo we talked about.

TXV is the Thermostatic Expansion Valve; it’s a restricted metering device. This device receives high-pressure liquid refrigerant and regulates the flow of refrigerant into the evaporator coil. In short, it’s the essential part of the air conditioner.

According to HVACR School for Techs, TXV is the most common metering device in modern air conditioners.

If you have a faulty TXV, the AC can stop working or run with limited BTU capacity. Here is a list of problems that may be caused by a bad thermostatic expansion valve:

  • Low evaporation pressure (not sufficiently high pressure).
  • Superheating of evaporator and compressor.
  • Low compressor amp draw.
  • Low-pressure control short-cycling.
  • Low condenser pressure.

As you can imagine, if the TXV doesn’t work properly, the chain reaction can cause the air conditioner to stop cooling or stop entirely.

How to fix an air conditioner with a failing TXV?

Again, simple. You replace it.

TXV-related problems are the most difficult ones. You will need an HVAC professional to help you out. Not only to replace the valve but to actually diagnose the problem with your air conditioner in the first place.

9. Are AC Refrigerant Levels Low?

Refrigerant leakage is one of the most common and more difficult reasons why air conditioners stop working.

Refrigerant levels or ‘freon’ levels should stay the same throughout the lifespan of the air conditioner (10-25 years). So how come freon levels could be low?

A leakage. This should be the fear of everybody who owns an air conditioner.

Leakage of refrigerant will decrease the levels of freon. If it gets too low, the air conditioner will seem to run normally, but it will start cooling.

In fact, low levels of refrigerant are the primary cause of AC not cooling. An HVAC technician can fix the leak and refill the refrigerant but it does take some skill to diagnose the AC leakage problem and to it entirely.

When It’s Better To Buy A New AC Vs. Repairing The Old One

Pretty much every HVAC technician is taught these 9 steps of how to fix an air conditioner. If you have checked them all out, tried DIY fixing your air conditioner, and the problems still persist, it’s maybe a time for a new air conditioner.

Especially older ACs can be tricky for repairs. More importantly, the new units have a higher energy-efficiency which can save you quite a lot of time. For example, you can check energy-efficient portable AC units here, window AC units here, and ductless mini-splits here.

All in all, many thanks to the HVAC group on Reddit for such a simple and useful insight in how they do their job.

2 thoughts on “AC Not Working: 9 Steps How To Fix AC (By HVAC Technician)”

  1. The only thing worse for our industry than working for free, is taking away another’s opportunity to make money. This article is both dangerous and irresponsible, as untrained individuals may hurt themselves or others, or void equipment warranties, by working with volatile energies like refrigerant under pressure and electricity. Call a professional.

    • Hello Ryan, thank you for your opinion. You’re right; these things can be very dangerous as we have explicitly written in the article. Calling a professional is a good option, yes; we hope our industry gets the much-needed appreciation as well.


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