AC Compressor (C) D Friedman 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.

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

Air conditioner compressor condenser unit (C) Daniel FriedmanIf 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.

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.

If the Compressor/Condenser Does Not Start At All

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 compressor motor start relay does not pull in (close) then it is not asking the compressor to run. Check:

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:

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 the MOTOR OVERLOAD RESET SWITCH

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.)


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.


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

Is the outdoor compressor-fan unit (COMPRESSOR & CONDENSING COIL) running?

AC Compressor (C) D Friedman

If not, be sure that the outdoor compressor/condenser unit service switch 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

  • An inefficient or damaged air conditioning compressor
  • Low voltage to the equipment
  • Improper refrigerant charge (so cooling coil is not sufficiently cool)
  • A compressor that has lost its ability to "compress" the refrigerant due to internal wear - This condition can be diagnosed by an air conditioning service technician who will install air conditioning manifold gauges onto the system to check the compressor suction vacuum and discharge or "high side" output pressure. If gauge ports are not installed on the air conditioner compressor unit the technician cannot make this test without cutting the refrigerant lines to install gauge ports (adding to the cost of this diagnosis).
  • Blocked or lost air flow due to a variety of causes (dirty filter, damaged or leaky ductwork) that fails to deliver enough cool air to satisfy the thermostat
  • Abnormal heat gain in the building (open windows, air leaks, unusual weather) so that the A/C or heat pump cannot satisfy the thermostat
  • Very low thermostat settings: if the thermostat is set too low the cooling system may not be able to bring the building down to that temperature
  • More diagnostic help for long cooling system on-cycles is at LOST COOLING CAPACITY


What about the opposite problem: too-short compressor on-cycles? See SHORT CYCLING AC COMPRESSOR


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:

  • The "repair" achieved by pouring water over a hot compressor is not lasting and if you wet electrical components it could cause damage or be dangerous
  • It's hot standing outside dumping water on your A/C unit and you're not supposed to have to do that
  • The loss of cooling means there is some problem to be found and fixed

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. COMPRESSOR & CONDENSING COIL
  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. REFRIGERANT LEAK DETECTION


Continue reading at COMPRESSOR & CONDENSING COIL or select a topic from the More Reading links shown below.


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