AC Compressor © D Friedman at Outdoor Compressor Condenser Unit Problem Diagnosis
Fix an A/C or Heat Pump not working, compressor won't start, condenser fan won' start, no cooling, short or long compressor on-cycles, compressor noises

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Compressor-condenser unit diagnostic procedures:

How to diagnose an outdoor air conditioning or heat pump compressor/condenser unit that is not working, won't start, noisy, or humming. This article summarizes common HVAC compressor condenser unit problems and their diagnosis. We link to more detailed HVAC compressor repair procedures.

This article forms part 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.

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.

Air Conditioner Compressor Problems - noisy or hard starting air conditioner compressors

Air conditioner compressor condenser unit (C) Daniel FriedmanArticle Contents

If the Compressor/Condenser Does Not Start At All

If your air conditioner or heat pump won't start, we suggest beginning diagnostics indoors at the room thermostat. If you haven't done that you might want to go back to

see AIR CONDITIONER WON'T START before continuing here. Below we give some summary diagnostics for non-starting compressor motors at the outdoor condenser unit.

Compressor problems - Air Conditioner Compressor: problems such as an aging air conditioner compressor motor that is at or near the end of its life may be unable to properly compress the returning refrigerant gas to a sufficiently high pressure. A service technician will need to evaluate and test the system and if needed, replace the compressor.

Since this is a costly repair, be sure to ask why the compressor failed and to correct any underlying cause (such as low voltage).

Variations in line voltage can lead to improper compressor operation and loss of cooling output.

Silent compressor/condenser motors: relays on the control board for the compressor/condenser unit start the compressor motor and the condenser unit's cooling fan individually, but they may be related as we'll explain. Check the outdoor components in this order:

1. Is there power to the compressor/condenser unit? Check that the main circuit breaker or fuse is on, that the outdoor service switch is on. If the compressor/condenser circuit's main breaker is tripped or fuse blown the system may have a seized compressor motor or a shorted electrical wire.

Back at AIR CONDITIONER WON'T START we reminded you to check that the thermostat was calling for cooling. If you forgot to do that, go back and check that article.

2. Is the refrigerant pressure in the system at the pressure level equivalent for the ambient temperature at the condenser unit? Modern compressor units include both high and low refrigerant pressure safety cut-out switches that can shut down the equipment in abnormal conditions.

3. Is the line voltage delivered to the compressor/condenser unit normal? Occasionally low voltage will prevent equipment from operating or will shut it down. Low voltage can also mean motors have trouble starting or run at abnormal power levels.

4. Is electrical power delivered to the compressor & to the condenser fan? Look for burned, shorted, disconnected wires supplying these two motors.

5. Is voltage being delivered to the start relay - the compressor contactor? If you see control voltage (typically 24VAC) at the contactor and you have line voltage to the compressor/condenser unit but the relay does not pull in then probably the contactor is bad (or there is a blown fuse on the control circuit board). Next we give diagnostic steps for the following situations


If the A/C or heat pump compressor motor start relay does not pull in (close) then it is not asking the compressor to run. Check the following:

If there is no control voltage at the compressor's start relay and there is electrical power to the unit and the thermostat is callling for cooling, then one of the safety cut-out switches we mentioned above may have tripped.

Check each of those safety relays for voltage on the relay output side. If there is no voltage then that relay has tripped (or failed).

Find and check the following:

Low pressure safety switch: if abnormally high pressures (typically over 55 psi) are found on the low-side of the compressor/condenser unit this switch will open to stop the system.

The low pressure switch normally will reset automatically when the low side pressure drops. If it does not re-set this switch has probably failed.

Condenser oil level safety switch: some compressor/condenser units (particularly larger and commercial systems) include a safety switch that monitors the oil level in the compressor motor.

Low compressor motor oil levels can shut down the system.

Watch out: most residential cooling systems use a hermetically sealed compressor motor; oil is not normally added tothese systems.

If you add oil to a commercial compressor/condenser system you need to check oil levels again after the system has returned to operation to be sure that oil levels are not too high - a condition caused when oil previously left in the piping system has returned to the compressor.

High pressure safety switch: The compressor/condenser over-pressure or "high pressure" relay safety switch.

A failed condenser unit fan motor or a damaged fan itself (motor spins but fan does not) can lead to compressor/condenser unit high temperatures and pressures.

If refrigerant pressrues in the system reach unsafe or abonormally high levels this relay will shut down the compressor motor. A clogged condenser or a failed thermostatic expansion valve can also cause abnormally high pressures in the system.

We first mentioned

at NOISES, COMPRESSOR CONDENSER that we had a field report from a reader who explained that a noisy outdoor compressor unit was, according to his HVAC service technician, traced to a blocked, clogged outdoor condensing coil. We speculate that perhaps the compressor was running hot and that correcting air flow across the condensing coil corrected that condition.

Compressor motor protection switch: some compressor systems include a protection module at the main power terminals for the compressor motor. This switch may have opened.

Voltage safety switch: The voltage monitoring or phase protector relay (not present on all equipment): transient low voltage can trip this relay. It may require manual reset.

Watch out: some electrical motor safety switches require a manual reset. For examples see this separate aritcle on


If the compressor motor start relay pulls in (closes) but the compressor does not run check:

Check for voltage at the compressor motor itself. If there is no line voltage check the main breaker or fuse or the local service switch again.

Is the compressor motor humming?: If you have a hard-starting air conditioner compressor that "hums" but doesn't start, it may be possible to get more life out of the compressor motor with a simple starting capacitor (rather than a costly whole new compressor motor.)


HVAC Condenser Fan Not Working

If the compressor motor will run but the condensing unit's fan does not run check:

Check that the fan blades will spin - with power off. If the fan motor won 't turn it's seized and needs to be replaced.

Check the fan relay switch: if the fan relay does not pull in and there is control voltage to the relay then the relay is probably bad. If there is no control voltage check the low voltage transformer and circuit wiring. Note: some systems incude a low ambient temperature control relay that will keep the system from running at low temperatures.

If the fan relay does pull in but the fan does not run, check the line voltage and wiring to be sure power is being delivered to the fan.

Check for voltage at the fan motor and go back to check circuit breaker or fuse if there is none

If spinning the fan will start the motor and it keeps running then the motor's start capacitor is probably bad.

If the fan motor has voltage but will not run at all then the motor may have failed or the motor's run capacitor may have failed.


Reader Question: outside [compressor condenser] fan does not turn on

(Aug 2, 2011) Raten said:

Outside fan doesnt turn on. What's wrong?



for an outside fan that does not turn on we give some diagnostic suggestions above in this


Start by assuring that the unit has power and that your thermostat is calling for cooling.

Your service tech may then check the fan relay, wiring, and fan motor, or the motor start/run capacitor.


HVAC Compressor Won't Stop

Question: My Air Conditioner won't turn off - what to do?

Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?


Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system.

If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician.

First check that the thermostat is not calling for cooling or turn it off completely. Also check for thermostat wires that are shorted togewther.

If the HVAC compressor motor will stop but the compressor / condenser fan is not working properly,

Air Conditioner Compressor Diagnosis: How to Diagnose & Repair an Air Conditioner Compressor Which has Lost Cooling Capacity

A compressor which appears to have lost cooling capacity can be diagnosed by a service technician who can connect the appropriate test gauges to the system. Lower than normal discharge pressure and higher than normal suction vacuum will indicate this problem.

But before assuming that something is wrong with the air conditioning compressor, some basic investigation is in order.

Unless there is an obvious indication of a compressor problem (noise, hard starting, compressor won't start), the service technician, to be thorough and economical, will inspect the system in an order, checking the easy and least-costly problems first, such as presence of electrical power, proper setting and operation of system controls, condition of filters, condition of duct work, operation of blower fans, before moving on to check the compressor itself by looking at the air conditioner operating temperatures, pressures, and current draw in Amps.

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 [photo] 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. A/C Compressor Problems

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 .

Cooling Capacity of the Air Conditioning Equipment

The cooling capacity of an air conditioning equipment refers to the ability of the compressor/condenser (usually outside) and the air handler/evaporator (usually inside) to deliver cool air to the occupied space.

Briefly, the compressor/condenser draws refrigerant gas from the building air handler, compresses it and cools it back to a liquid refrigerant, and the air handler/evaporator coil permits liquid refrigerant to evaporate inside a cooling coil, across which the fan blows building air to cool and dehumidify it.

The particular combination of this equipment has a cooling capacity, usually rated in BTUh or thousands of BTU's of cooling capacity per hour, documented on equipment data tags discussed


Also see COOLING RULES OF THUMB to guesstimate how many tons or BTUs of cooling a building needs

Long On-Cycles: Air Conditioner or Heat Pump Long on cycle and Insufficient Cooling - Loose or Worn Compressors

A longer than normal on cycle combined with little or no cool air conditioner output could be due to

Short HVAC Compressor On Cycles

What about the opposite problem: too-short compressor on-cycles?


Here is a brief summary of some diagnostic tips for short cycling HVAC compressors:

The outdoor condenser unit or compressor motor keeps cycling on and off too often

A/C system starts then shuts off

(June 8, 2012) Renee said:

My AC unit during the hot and humid months here in FL, will not run without some help from me. Lets say it is 80 degrees and you set the thermostat at 75, it will run until temp drops about 3 degrees, then shuts off.

You then need to shut the unit off via the thermostat, for approximately 20-30 mins turn it back on in order for the air to start blowing again. I tried changing out the thermostat but this has not helped. What could be the problem?


Renee, often when shutting off the A/C returns the delivery of cool air we trace that problem to frost formation on the cooling coil.

Click on FROST BUILD-UP on AIR CONDITIONER COILS check for, diagnose, & repair that problem. But first, make sure that your air filter is not debris clogged.

My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems

Joe said:

Short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!

Matthew said:

Short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old

Five causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair

(mod) said to Joe and Matthew (re questions just above):

  1. Loss of air conditioner refrigerant - a refrigerant leak in the system. You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak. Otherwise you will have to keep repeating the costly service call to just add refrigerant.|

  2. A/C Coil Icing - the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.

  3. Oversized air conditioner - if the short cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled.

    A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled.

    Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat.


  4. A/C control problem - it's less likely, but a damaged control board or switch could also be causing rapid equipment on-[off cycling.
  5. Compressor damage or compressor start-troubles: - I've seen these other causes of air conditioner short cycling:

    if someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor.

    Normally that head pressure bleeds off over time, making it easier for the compressor to re-start (against low head pressure) the next time it turns on.


But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.

We have also see or a damaged compressor internal refrigerant valve causing high head pressures;

In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.

If your air conditioner or heat pump has the opposite problem, staying on too long,


Low air conditioner motor amperage draw

Low amperage draw: unlike a high-amp current draw which indicates that the compressor is danged internally in a way that its piston(s) is(are) tight in the cylinder, a low-amp current draw, if below normal, may confirm internal wear on the compressor parts, and would support the diagnosis that the compressor is worn and inefficient.

Where there are no gauge ports to actually measure compressor low side and high side vacuum and pressure, this simple electrical test is a useful first step.


If you have the opposite concern, that is the A/C compressor is turning on and off too frequently,


Abnormal air conditioner compressor pressure readings

Refrigerant line pressure readings which are abnormal (probably too low) on the high pressure side (compressor output) or on the low pressure side (compressor input or suction line) can indicate a problem with the compressor's ability to develop normal operating pressure ranges and thus will affect the cooling capacity of the air conditioning system.

For more details on air conditioner refrigerant line pressures and how they are examined


Details about air conditioning compressor functions, how air conditioner compressors work, what goes wrong with air conditioner compressors, and how to measure and diagnose air conditioner compressors are provided


Compressor/Condenser Cooling: Can Cooling Down the Air Conditioner Compressor Motor with Water Keep the A/C Working?

Several readers have described a step of cooling down an overheated air conditioning or heat pump compressor motor to get it running again producing cool air indoors. One reader used a garden hose to spray the outdoor unit (after all it's intended to tolerate being rained-on). Another used a watering can. A third, buckets of water.

All three reported that this step got their cooling system cooling again when before either the compressor had stopped running entirely or it was running but the system was not producing cool air. But for several reasons this is not an effective durable repair:

While we wait for more comments from air conditioning service technicians and experts (CONTACT us), here are some interpretations of what may be going on:

  1. The A/C compressor motor may be at or near end of life, running hot. An air conditioner motor that is overheating may turn itself off on thermal reset. In this case when the motor cools down it may automatically reset itself and start up again - for a while. Sometimes you can get a compressor motor starting again on its own by adding or replacing a starting capacitor -



  2. The A/C compressor motor may be overheating due to its location: for example in hot sun. Most equipment will tolerate this condition but an older unit or very hot sunny conditions may lead to thermal overheating and an overload circuit trip-off.
  3. The A/C system compressor/condenser fan may not be running or may be damaged, failing to adequately cool the refrigerant gas in the condensing coils. Watch the fan to see that it is running while the compressor motor is on, and perhaps even before and after the compressor motor starts or stops. [But not indefinitely.]
  4. The A/C condenser unit coils may be blocked, bent, dirty, damaged. It may be possible to clean the coils of leaves, dirt, grass kicked up by nearby mowing, etc. and thus improve the condenser operation and start making cool air again.

  5. The air conditioning or heat pump system may be very low on refrigerant. You'll need a service technician to check and repair this condition.


Basic Compressor Condenser Unit Wiring Connections

Contactor relay T1

Red wire to dual start/run capacitor C or COM terminal

Red wire to Compressor’s R run terminal

Red wire to Fan motor’s R run terminal

Contactor Relay T2

Black wire to Compressor’s C or COM terminal

Black wire to Fan motor’s C or COM terminal

Contactor Relay L1 - black line voltage in

Contactor Relay L2 - red line voltage in

Contactor Green wire connects grounds contactor & other components to system ground

Contactor Relay low voltage wires (24VAC) to the magnet that operates the relay

Start-Run Capacitor FAN terminal yellow or brown wire to Fan motor’s S terminal

Start-Run Capacitor C or COM terminal red wire to Compressor/Condenser Contactor Relay T1

Start-Run Capacitor H or HERM terminal purple wire to Compressor’s S or Start Terminal


Continue reading at COMPRESSOR / CONDENSER REPAIR - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see COMPRESSOR / CONDENSER DIAGNOSTIC FAQs questions and answers posted originally at this page




Or see these

HVAC Compressor / Condensing Unit Diagnostic Articles

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