AC/Heat Pump Troubleshooting Guide

This ac/heat pump troubleshooting guide assists you in diagnosing what is wrong with the AC and whether the problem is a DIY fix or professional repair. When do-it-yourself central air conditioner repair is possible, we provided tips and videos to assist.

Once you determine the problem, visit our Central Air Conditioner Repair Cost Guide for an estimate of the professional repair price, information on repairing vs. replacing the AC and finding a qualified, dependable repair or installation professional.

Troubleshooting Common Air Conditioner Problems

Before you call the AC repair service, use these AC troubleshooting tips that address common problems, their causes and cures.

1. Heat Pump/AC Not Working

If the temperature in your home is higher than the thermostat is set to and the thermostat is on AC mode, then check for these problems in this order:

  1. Check the AC circuit in your electrical panel and the one outside on the all near the AC condensing unit, and if either is off, turn it on
  2. If the unit continues to trip the circuit, there is a short somewhere, and a technician should diagnose and repair the issue
  3. If the circuits aren’t the problem, try flushing the drain line DIY as in this basic video (and the parts to make the tool are available at your local home improvement store) or this advanced video or have the drain pan cleaned and the line flushed by an HVAC technician
  4. If the circuits are on, the line is clear and the AC still won’t start, the condensation pump, if your unit has one, might not be working, tripping a limit switch that prevents the AC from starting, and the pump will require replacement
  5. Check the circuit control boards in the furnace/air handler for flashing LED codes that indicate the problem with the use of a code key on or near the board
  6. If the control board code indicates it needs to be reset, turn off the furnace/air handler circuit for 30 seconds before turning it back on
  7. If this problem persists, the circuit board likely needs to be replaced, and this can be done DIY or by a technician who will ensure that is the right repair

DIY Tips: If you replace a control board yourself, and it is not the solution, you probably won’t be able to return the board, so keep that cost risk in mind as you consider DIY vs. professional AC repair. Before replacing the board, take a picture of the wiring connections or label them with tape (or both) to make sure the wires are connected to the new board the same way they were connected to the original board.

2. Condensing Unit Fan Running; the Indoor Fan Not Working

If the outside unit starts, but your furnace or air handler isn’t blowing air, then:

  • Test the blower motor using this guide
  • Inspect the capacitor, and if it is leaking oily fluid, is bulging or shows burn marks, it has failed and must be replaced
  • Test the run capacitor using a multimeter after watching this video, and determine if it is getting power and has a reading within 5-7 percent of the rating listed on the capacitor, and if it isn’t, the part is burned out and must be replaced
  • Test the furnace/air handler control board as discussed in the previous repair to see if it needs to be reset or replaced

Burned out capacitor from Source: DIY Chatroom

DIY Tip: Keep in mind that these tests require the use of safety precautions, electrical testing tools and experience, and the job is best left to an HVAC technician.

3. Heat Pump/AC Not Cooling as Expected

When your AC and blower are running but your home is still warm:

  • Make sure the thermostat hasn’t been turned off or adjusted to a higher temperature than you want it to be
  • Clean or replace the furnace filter if needed, because a dirty filter will reduce the cooling power of the system
  • Clean the coil in the outside condensing unit by removing the AC cabinet, gently brushing the coil (radiator-type fins) with a nylon or natural bristle brush and hose off the debris
  • Clean the inside coil, if you can access it, with a soft brush and/or a shop vacuum using the brush head
  • OR, call an HVAC professional to clean your coils and give the system a tune-up

If these aren’t problems, then the system is likely low on refrigerant, and an AC technician will need to look for a leak, repair the leak and recharge the system to the proper level of refrigerant

Homeowner Tip: Most HVAC companies offer service plans that include cleaning the coils once or twice per year and other maintenance steps that can keep your AC running efficiently and durably while also preventing some costly repairs. Some include a discount on the types of repairs we’re discussing here and priority service if your AC fails. HVAC maintenance contracts and whether they are worth the cost are discussed in our AC Repair Cost Guide.

4. The Furnace or Air Handler is Leaking Water

There are three potential causes for this malfunction. The first was discussed above – the condensate drain being blocked and requiring flushing. Let’s address the second possible cause:

  • Check for a blockage of the drain pan hole leading to the drain line, and clear debris, algae and sludge from it to allow water to flow – and flushing the drain following this procedure is a good idea too
  • If this isn’t the issue, then consider the third reason for a leak, AC coil freeze-ups, discussed next

5. AC Coil Freeze-ups

A small amount of water on the floor near the furnace or condensation on the furnace cabinet might indicate the indoor coil has frozen. This occasionally happens in very hot, humid weather or if you’re running the AC while outdoor temperatures are below 60F.

  • Remove the furnace cabinet door or cover to see if the indoor coil is iced over, and if it is, turn off the AC and set the blower to “fan only” mode to allow warm air to pass over the coil until the ice melts
  • Once the ice melts, check the indoor coil, and clean it or have an HVAC tech clean it if it has built-up dirt and debris
  • Clean or replace the furnace filter, because restricted air flow will cause the coil to get too cold

Repair Tip: If these solutions don’t work, you likely have a kinked refrigeration line or the system is low on refrigerant. Another cause might be that your blower motor is failing or very dirty and not running at full capacity. These causes are best diagnosed and repaired by an ACtechnician.

6. The AC is Over-cooling your Home

The most common cause of this is that the thermostat control is too close to a lamp or electronics that are giving off heat, and the thermostat constantly thinks it is warmer in the house than it actually is. If some parts of your home are cool, but it is warm near the thermostat, the cause might also be that the registers near the thermostat are closed or otherwise blocked. If those things aren’t the issue, then replacing the thermostat control should solve the issue.

7. System Making Unusual Noise

Central air conditioner condensing units make a starting noise and then hum through their cycles, often making a clicking noise when shutting down. All these are normal. The following noises aren’t, and here is what they mean:

  • Low, constant humming with the fan not running: Fan motors that seize up and need to be replaced often make a low humming noise, though the issue could be a failed compressor too
  • Vibration hum: This type of humming often indicates something loose in the condensing unit such as the fan motor mounts or the cabinet, and tightening the fasteners or placing foam rubber between rubbing parts can stop the noise
  • Sharp buzzing noise: The condensing unit will buzz at the start, but if the buzzing persists and the AC won’t start, it indicates a bad start relay or capacitor
  • Squealing: A bearing in the condenser fan (if the noise is outside) or the blower motor (if the noise is indoors) is probably bad and should be replaced before it comes apart, potentially causing major damage to the unit
  • Whistling: The most common cause of high whistling noises is debris clogging the thermostatic expansion valve (TXV)
  • Squealing accompanied by clanking: This issue can be caused by a loose or worn pulley on a belt-driven system (moderate repair) or might indicate the compressor is failing (major repair or replacement)
  • Screaming or high-pitched hissing sound: This indicates refrigerant pressure in the system that is high enough to cause the compressor to explode, so power to the unit should be shut off immediately, and a service technician should be called
  • Rattling: Your AC might have a hard-starting issue, and that is easy to solve with the right part, or the compressor might be starting to fail, and that often means it is time for a new central air conditioner

8. Heat Pump Not Heating

We already wrote an article about this problem:

9. AC/Heat Pump Outside Unit Not Running

We already wrote an article about this problem:

Hire your HVAC Technician Carefully

Maintenance, repairs and replacement of an AC/heat pump bring the best value when they are done properly. Therefore, it makes sense to get written estimates from several of the top HVAC professionals in your area to find not just a fair price but also a company with a track record of quality workmanship backed by parts and labor warranties. Our Free Local Quotes tab puts you in touch with some of the best technicians in your area, and there is no obligation or cost to you.

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