AC Compressor © D Friedman at Air Conditioner / Heat Pump Diagnostic Procedures
How to Diagnose an A/C or Heat Pump not working, won't start, not cool, weak airflow, noises, etc

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Air Conditioner or Heat Pump Diagnosis:

This article is the start of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperaturer, or an air conditioner that won't start.

First try our air conditioner checklist just below - try these steps before calling your HVAC service company.

If not enough cool air is provided by your air conditioner, or if the air temperature is not cool enough, or if you just can't get your A/C unit running, this article helps diagnose and correct the problem with step by step things to check and links to more detailed explanation when you need it.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What to Check First if the Air Conditioner Output is Inadequate or A/C Won't Run

Air conditioner air not moving © D Friedman at

Article Series Contents

These Simple Checks May Enable Inexpensive Repair of Lost Cooling Output from an Air Conditioner

If you don't see information you want, ask us for it using the comments box On this page.

If your air conditioner or heat pump is a split system or mini split system design (using a wall-mounted cooling or heating unit) our troubleshooting diagnostics are at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS.

Is there no cool air at all coming out of the supply registers?

Or is there air blowing out of the supply registers but it's not cool enough?

Here we explain how to diagnose loss of air conditioning cool air flow or cool air temperature.

Before ordering an expensive air conditioner service call to restore lost cooling capacity, here are a few simple steps to perform. Some of these can be done by any homeowner, others may require a bit more expertise.

Below we provide first a very basic Air Conditioning or Heat Pump Diagnostic Checklist, followed by more detailed descriptions of common problem cooling system topics.

Following the checklist we provide more detailed descriptions, photos, and links to in-depth air conditioning inspection, diagnosis, and repair or improvement advice.

Where to start: check that the air conditioning equipment is turned on, the thermostat is calling for cooling, and that the blower unit or air handler is actually blowing air through the ductwork. Details of how to proceed in figuring out what's wrong with your air conditioner begin just below.

Air Conditioning Checklist for Lost Cooling: What to Do First in Air Conditioner / Heat Pump Tests

If the air conditioning system is not running at all:

Air conditioning thermostat (C) Daniel Friedman
  1. Check the Room Thermostat Temperature Setting: Set the thermostat to at least 5 degrees below room temperature. Our elderly mom has no patience with switches and controls. She regularly calls her air conditioning service company with a service request, sometimes late at night, because she has simply failed to set the temperature on the thermostat lower than the room temperature. Don't drive your A/C like our mother.
  2. Check that the Room Thermostat is set to "Cool" not "Off" or "Heat". If the thermostat is not set to "cool" it is simply turning off your A/C.
  3. Check that electricity is on for the equipment. Check all switches and controls, including service switches, including outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel.

    For example, condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner. There are several switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch, an electric motor overload or overheat switches, and a condensate tray spillage detector switch.

    Watch out: See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system. A bad or failed starter capacitor could also be leaving your system shut down, failing to start a blower, fan, or compressor motor. Some hard-to-find switches could be keeping your air conditioner from starting, such as

    a Float Switch on Condensate Tray or


If the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply registers you probably need to call a service technician, but below are some things to check further yourself.

If At Least Some of the Air Conditioner or Heat Pump Equipment is Running but Cool Air is Not Being Delivered - 5 Quick Diagnostic Air Conditioning / Heat Pump System Checks to Get the Air Conditioner Running Again

The basic air conditioning diagnostic checks just below are followed by more detailed investigation of why the air conditioner is not working. But unless you already have a good idea of what's not working, start with these five steps.

If the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply registers you probably need to call a service technician, but here are some things to check further yourself.

Just after these checks we discuss what to do if the air conditioner is running but air flow is too weak - A/C FLOW TOO WEAK

or what to do if the air conditioner or heat pump runs but in cooling mode the air supplied is too warm - LOST COOLING CAPACITY

Step 1: Check the Outside Air Conditioning /Heat Pump Compressor-Condenser unit

AC Compressor © D Friedman at Is the outdoor compressor-fan unit (COMPRESSOR & CONDENSING COIL) running?

If not, be sure that the outdoor compressor/condenser unit service switch [image] and the air conditioner indoor power switch, fuse, or circuit breaker are all in the "on" position.

Check the outside compressor condensing coil for damage or blockage.

A blocked condensing coil can cause the compressor to overheat, stop running, or become noisy and work poorly.
See A/C COMPRESSOR PROBLEMS - diagnostic aricles for the compressor/condenser unit.

If you cannot find your air conditioner controls and switches
see A/C - HEAT PUMP CONTROLS & SWITCHES - list of controls and switches found on air conditioners and heat pumps

Also check the compressor/condenser outdoor fan and fan motor

Condensing unit fan diagnostics:
at FAN, COMPRESSOR / CONDENSER UNIT we explain the diagnosis of problems with the compressor/condenser fan and fan motor.


at BURNED-OUT COMPRESSOR we explain that if the condensing unit fan is not working the compressor itself may shut down or even be ruined by overpressure or over temperature.

Also see NOISES, COMPRESSOR CONDENSER where some noise problems are traced to the cooling fan .

Step 2: Check the Air Conditioning /Heat Pump Indoor Air Handler Unit

Air handler © D Friedman at

Is the indoor blower unit running? If not be sure that the electrical power switch at your furnace or air conditioner air-handler is in the "on" position.

Typically the air handler or blower unit is indoors inside the basement, crawl area, or attic.

Sometimes there is more than one switch, such as one right at the unit and another at the entry to the room where the equipment is located, or even upstairs on a higher floor if your air handler is in a basement or crawl space.

Make sure that the blower compartment cover or door is properly closed or an interlock switch may be keeping the system "off".

If the blower unit fan is not running or runs slowly, see BLOWER FAN OPERATION & TESTING.

If the blower runs normally but air flow is warm or weak, remember to check for a dirty, blocked COOLING COIL or EVAPORATOR COIL.

Also see ELECTRIC MOTOR DIAGNOSTIC GUIDE if you suspect a blower motor problem.

Step 3: Weak air flow: Check the Air Conditioning /Heat Pump Air Filter(s)

AC Filter dirty clogging © D Friedman at

If your air conditioning equipment is running but little or no cool air is coming out of the supply registers, check that your air filter(s) have not become blocked with dirt and debris.

Usually the air filter is right at the air handler or blower unit, or your air filter may be installed behind a grille covering a central warm-air return that sends air back to the air handler.

See A/C FILTER PROBLEMS for details as dirty air filters are a common cause of weak air flow and inadequate cooling.

If the air conditioner or heat pump is running but air flow is still too weak you may want to see our list of causes and cures for weak air flow at A/C FLOW TOO WEAK

Step 4: Weak air flow: Next Check the Air Conditioning /Heat Pump system duct work & air supply registers

Air supply register leak © D Friedman at

In ceilings, walls, or floors, where cool air is supposed to be delivered to various rooms in the building, be sure that the register is in the "open" position (you will be able to see through it into the ductwork) and be sure you haven't blocked the supply registers with a carpet or furniture.

Look for a duct damper or register that has been closed; look for a flex-duct section that has become disconnected, bent, or crimped or squashed. Remember that a duct may have become disconnected in an attic or crawl space.

Look for an air filter that has come loose and blown into the ductwork, clogging it.

In our photo at above/left you can see a ceiling air supply register that has leak stains around its opening - further investigation for a condensate leak or roof leak is needed.

See DUCT & AIR FLOW PROBLEMS for a complete diagnostic guide to inadequate air flow problems

Also see RETURN AIR, HVAC - inadequate return air will reduce the supply of cool air in cooling mode or warm air in heating mode.

Step 5: No air flow at all: Check the Air Conditioning /Heat Pump indoor air handler blower fan assembly

Photograph of attic air conditioning air handler, condensate drips on floor

If the indoor air handler blower fan itself won't start, see BLOWER FAN OPERATION & TESTING. Check for lost power, a fan motor that wont' start or has tripped a motor reset button or needs a start / run capacitor.

Check for a blower fan that is loose on the motor drive shaft or for a broken or lost blower fan drive belt on pulley-driven blower systems.


Step 6: Air flow is OK but air is not cool: check for lost refrigerant, a stuck refrigerant metering control, a blocked or iced up cooling coil

AC Compressor © D Friedman at

A coil blocked by ice or dirt will not produce cool air A/C COOLING COIL ICING. The coil may need cleaning, or low refrigerant or a bad expansion valve may be causing ice blockage on the coil.

If the air conditioner or heat pump runs but in cooling mode the air supplied is too warm see LOST COOLING CAPACITY

where we discuss the things that will cause warm air discharge from the supply registers of an air conditioning system or from a heat pump system in cooling mode.

Examples include:

Major Areas to Investigate for Problems With Air Conditioning Systems & Equipment



Continue reading at AIR CONDITIONER WON'T START or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see these

HVAC repair guides

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