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How to Repair a Central Air Conditioning System

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1. Troubleshooting a Central Air Conditioning System

What happens when your central air conditioning system suddenly has little to no cooling power? This tutorial will show you how to troubleshoot and repair your air conditioning system so you can hopefully stay cool without a service call. A home air conditioning system includes five main elements:

  • Refrigerant
  • Compressor
  • Condenser
  • Expansion valve
  • Evaporator Coil

In addition, there is the “brain” of your heating and cooling system, the thermostat, which can also be an issue at times. Let’s take a look at some common air conditioning problems and possible solutions. Note that the focus here is central air conditioners, a window air conditioner has similar components in much smaller form and involves different troubleshooting techniques.

2. PROBLEM: Air Conditioning System Will Not Turn On

Possible Causes

If you cannot get the air conditioning system to activate at all, then the most common causes lie with a tripped circuit breaker or blown fuse, an improperly set or faulty thermostat, or an internal switch being off.

Possible Solutions

Ensure the thermostat itself is in the “Cool” position and not set to “Off” or “Heat.”
Confirm that the thermostat is set below the current room temperature.
Check that the 240-volt circuit breaker (double-pole breaker) controlling the air conditioning compressor/condensing unit and the 120-volt circuit breaker controlling the furnace blower or separate air handler, are in the ON positions. If a circuit breaker has tripped or a fuse is blown, then reset the circuit breaker or replace the fuse. If you reset the breaker or replace the fuse and they trip or blow again, stop and call an air conditioning service technician, as you may have a more serious problem.
Confirm that all switches in and around the air conditioner are set to the ON position including the external safety switch, which is usually on an outside wall next to the condensing unit.
Check the condensate overflow tray (if your unit has one) for excessive water. Sometimes this tray is installed in remote air handlers using condensate collection instead of a condensate drain. When using a tray, there may be a sensor switch that turns the unit off when water collects in the tray.
Make sure the blower door on the air handler is securely closed.

3. PROBLEM: Poor Airflow from Cooling Vents in Rooms

Possible Causes

Poor airflow usually results from a dirty air filter or ductwork that has become blocked, crimped or even disconnected.

Possible Solutions

Confirm that the air filter in the air handler is clean. If it’s dirty, clean or replace the air filter.
Visually inspect all ductwork to make sure it has not become disconnected or crimped. This includes ductwork that may be in hard-to-reach attic, basement, or crawl space areas. Repair or connect ductwork as needed.
Check register dampers on vents in the rooms to make sure they are fully open.

4. PROBLEM: Air Conditioner Does Not Dehumidify Air—Oversized System

Possible Causes

This problem can be caused by an air conditioning system that is too large for the house. If your system has always failed to dehumidify adequately, and it commonly short cycles (turns on and off frequently), an oversized system is a likely cause. An oversized system can cool the air so fast that the thermostat shuts off the system before much of the humid air has made its way across the cooling coil to get dehumidified.

Possible Solutions

The only real solution is to replace the condensing unit and coils with a properly sized system. You can also try to increase the cooling load artificially, such as by air conditioning your garage or basement space with additional ductwork.

5. PROBLEM: Air Conditioner Does Not Dehumidify Air—Properly Sized System

Possible Causes

Poor dehumidification can also be caused by excessive humidity in the home. This may be caused by water leaks or open windows during humid periods or by a lack of proper condensate drainage from your evaporator coil in the furnace’s plenum. A properly functioning cooling unit produces condensate and drains it away. If your unit is not producing condensate its cooling and dehumidification functions are not working properly.

Possible Solutions

Ensure all windows are closed so humid air does not come into the home.
Check that condensate drainage is working properly.
Add supplemental dehumidification with a portable dehumidifier.

6. PROBLEM: Inadequate Cooling with Long Compressor Cycle

Possible Causes

Inadequate cooling coupled with a long “on” cycle of the compressor is a sign of a worn-out compressor that has lost its ability to compress refrigerant.

Possible Solutions

Have a service technician test the compressor and possibly replace the part.

7. PROBLEM: Short Compressor Cycle

Possible Causes

A short-cycling compressor—one that turns on and off frequently—can be caused by an obstructed thermostat, leaking refrigerant, iced coils, or, in some cases, an oversized cooling system.

Possible Solutions

  • Ensure the thermostat is not obstructed.
  • Check the condenser fins in the outside condensing unit to make sure they are not dirty or excessively bent; repair bent fins with a fine comb.
  • Replace the air filter in the air handler if it is dirty.
  • Have a service technician check for incorrect refrigerant charge or refrigerant leaks.
  • Replace an oversized condensing unit and coils (requires service technician).


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