Air conditioning troubleshooting is an essential process to identify the exact servicing that your clients need. A proper troubleshooting process will ensure not only efficient servicing, but a quality customer experience that will increase the likelihood of repeat business. While troubleshooting can be done by just talking to the customer over the phone, possibly saving an on-site review, the process usually involves both information collection and direct inspection of the unit.

In this article, we’ll review the most commonplace problems that technicians encounter in the field, and will include an air conditioning troubleshooting chart that can help with the diagnosis and servicing. 

What to know before answering the call

There are important things to ask the HVAC or AC owner before reviewing any further: 


  • Is the filter dirty? A dirty HVAC filter can be the culprit of all kinds of problems, from poor air flow to poor performance overall, a drop in indoor air quality, short-cycling of the system, a spike in utility bills and even iced-over evaporator coils. Even the right choice of HVAC filter makes a big difference, since HEPA filters and other more sophisticated designs can be restrictive enough to cause a drop in airflow through the system. Make sure that the HVAC unit is working properly through spring HVAC cleaning and inspections.
  • Is your thermostat set to Cool and not Heat? Seems like a no-brainer, but you’d  be surprised how many times it happens. 
  • Is the main breaker for your system tripped? Surprising, but this is often overlooked and you have to ask anyway because it could save you a wasted service call. Make sure that the customer checks the breaker for the furnace as well, because that can be the root of the problem too.


  • Is the outdoor fan running? An outdoor condenser fan that’s failed (for whatever reason) can be enough to cause trouble. 

Common problems (and their solutions)


  • High energy bills
  • Poor airflow
  • Poor performance in general
  • Ice on refrigerant lines or evaporator coils

Potential fix: 

Our first suspicion here should be the air filter. If the filter is clogged and saturated with debris, dust or pet fur, then go ahead and replace it. Remind the customer that the filter should be replaced every 30 to 90 days, and that marking the day on a calendar is a great way to remember. 

Poor airflow through the system can be enough to drop the temperature of the refrigerant, causing the coils and lines to ice over. There are other problems that can cause iced-over coils, though, and we’ll touch on those in a minute. 


  • System performs poorly with a loss of cooling capacity
  • System runs almost nonstop
  • Coils and lines ice over while system is running

Potential fix:

Check the evaporator coils for a buildup of dust and debris. If the coils need to be cleaned, be very gentle with a spray hose since the fins can be bent and damaged easily. Tools are available for cleaning (and straightening fins if necessary), and commercial-grade foaming solutions can be used to dissolve heavy buildups of dust and gunk. 


  • Loss of cooling capacity 
  • Coils and lines ice over while system is running

Potential fix: 

If the evaporator coils and AC filter look to be well-maintained and clean, the problem might be low refrigerant levels. Remember the law of thermodynamics: low refrigerant pressure means the system will absorb more heat than it should, making the lines (and then the coil) ice over.  If refrigerant has migrated out of the system, you’ll also need to perform leak detection and repair as well as recharging. 


  • System won’t start at all

Potential fix: 

The system’s start capacitor can be the culprit, and it’s a component that fails on HVAC systems pretty regularly. Sometimes the start capacitor will fail suddenly, but other times it might make a loud humming noise, smoke or a burning smell, the system might run but not blow cold at first, or the HVAC might shut off at random times. 

When diagnosing a start capacitor problem, start with a visual inspection first. You might notice cracks, bulging, a leak of liquid from the capacitor and circuit board or burned contacts. Check the capacitor with a multimeter and be mindful of the voltage it can store! Since we’re on the subject, remember that you should never, ever troubleshoot air conditioners unless the system’s power is shut off and the system is de-energized. 

TIP: if you’re unsure how to do the replacement, use your phone to take a picture of the existing capacitor and its connections before you get started. 


  • Outdoor fan won’t run

Potential fix: 

This could be either our old friend the start capacitor or the fan motor itself. Try spinning the fan blades with a stick to see if the motor will kick in (don’t use your hand!); if it takes a jump start like that to get it going, that’s a pretty good sign that it’s the motor. Don’t forget to check the motor’s connections as well as the outdoor circuit board and controls; insect infestations or rodent damage are pretty common causes of HVAC problems too. 

An inoperative outdoor fan is a real problem that needs to be addressed right away. If the fan can’t exhaust hot air from the unit, the compressor can quickly overheat and potentially run up the bill a lot more. These problems are, of course, aggravated by extremely hot weather. 


  • AC system keeps tripping the breaker

Potential fix: 

There are all kinds of reasons why this could be happening, ranging from pretty minor causes to much more involved ones. Start with the minor ones: 

  • Check the wiring for loose connections, damage or corrosion
  • Short circuit
  • Rodent damage to wiring
  • Dirty filter
  • Dirty coils
  • Outdoor condenser fan malfunctioning


  • System won’t kick on
  • Loud humming
  • Loud chattering

Potential fix: 

These symptoms point to a faulty contactor, the component that actually enables the compressor, exhaust fan and everything else to start working and doing its job. The contactor might have stopped working due to corrosion or a buildup of dirt at its electrical connections, which is easy enough to remedy with some aerosol contact cleaner. If that’s not the case, check the continuity of the contactor using your multimeter and replace if needed. The contactor generally has a service life of around five years on most systems, and is a fairly inexpensive part to replace. 

Tools needed

Here’s a rundown of the tools a tech typically needs on a service call: 

  • Adjustable wrench
  • Cordless screwdriver
  • Insulated screwdrivers (Phillips and standard) in different sizes
  • Nut drivers
  • Socket set and ratchet
  • Multimeter and voltage tester
  • Needle Nose pliers
  • Diagonal cutters
  • Line gauges for refrigerant levels

Be patient

Like we said at the beginning, anytime you troubleshoot air conditioners you should be methodical about it. Use your training and your experience, start with the simplest and most obvious things first and work your way through potential causes from there. Don’t get ahead of yourself and don’t jump to conclusions! 

You’ll also need to be patient about restarting the system to check its performance after the repairs are completed. For safety, thermostats and HVAC systems are engineered with a feature that can delay operation by as much as ten minutes if the system is shut down, de-energized, then repowered and started. If the system is subscribed to an energy-saving setup with the local utility, this restart can take even longer. If the system isn’t performing up to par after 30 minutes, though, it’s safe to assume something’s not right and you may need to start the diagnosis/repair again. 

There’s a lot at play in an HVAC system but these are the most important things to know when learning how to best conduct HVAC troubleshooting.  While this rundown of commonplace problems that techs run into while on the field, don’t forget to ask the homeowner the questions we mentioned at the start of this article. There’s a chance you won’t need to make the service visit if technicians are at capacity during the busy season.

Additional Help

If you have any questions or would like to consult with our team, we’re here for you. We strive to go above and beyond to service our customers. We dedicate ourselves to fostering close, ongoing relationships in order to help customers grow their business. While offering only the highest-quality HVAC equipment and parts, we can provide you with the advice and expertise you need. Contact Us!

NuComfort LLC is a wholesale distributor of both commercial and residential HVAC systems. We also stock a full line of accessories and replacement parts. We have a long history of satisfied customers, and are ready to serve you from our three Chicagoland locations in Glendale Heights, Chicago and Crestwood.