What to check if your air conditioner or heat pump just won't start at all. Here we give a diagnostic sequence of things to check if the A/C is just not working.

Checking these simple items, switches and controls, including some you may not know about (overflow pan switch, for example) may get your system running without a costly service call.

This article is part of our extensive series on how to diagnose an air conditioner or heat pump that is not cooling.

What to Check First if the Air Conditioner or Heat Pump Won't Start

Is the indoor air handler blower unit or fan unit motor running or is it silent?

If it's not running, either the thermostat is not calling for cooling or there's a control or electrical or motor problem at the air handler.

If you hear the blower motor running at the air handler but no air is coming out of supply registers, check for a broken blower fan belt or similar problem. That's not a "blower unit won't start" issue.

Is the outdoor air conditioner or heat pump compressor/condenser unit motor and/or cooling fan silent?

If silent, it's not running.

If the outdoor compressor motor runs and the compressor unit fan runs but inside the building the supply air is not cool, we suspect a refrigerant or refrigerant control problem. That's not a "compressor won't start" issue.

Here are the things to check if your air conditioner or heat pump doesn't start at all when you set the room thermostat to call for cooling:

Step 1: Check the Room Thermostat Temperature Setting

Has the cooling thermostat temperature been set below ambient room temperature?

Set the room thermostat to at least 5 degrees below room temperature.

My elderly mom had no patience with switches and controls.

She regularly called her air conditioning service company with a service request, sometimes late at night, because she simply failed to set the temperature on the thermostat lower than the room temperature. Don't drive your A/C service technician mad like my mother did.

Step 2: Check that the Room Thermostat is set to "Cool" 

Be sure the thermostat mode switches are not set to "Off" or "Heat".

If the thermostat is not set to "cool", it is simply turning off your A/C.

If the thermostat display is blank, then it's not receiving power (for modern digital thermostats). Check that electrical power is on at the air handler and to the the low-voltage transformer that supplies power to the thermostat.

If the thermostat has power, then check for broken or shorted thermostat wires anywhere between the wall thermostat and the control board at the air handler.

Tip: You can easily eliminate possible thermostat problems as a cause of failure of the air conditioner to start by simply eliminating the thermostat from the picture: disconnect the thermostat wires at the blower unit's control board and instead connect the two thermostat terminals directly together with a jumper wire. If the system starts then the problem is in the thermostat itself or in its wiring.

See THERMOSTATS for a details of how air conditioner thermostats work and how the air conditioner blower fan controls work.

Step 3: Check that Electricity is Turned ON for the A/C Equipment indoors & outside

Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?

Has the air conditioning electrical wiring been physically damaged or cut? Our photo above shows damaged electrical wiring at the outdoor compressor/condenser unit, courtesy of Tim Hemm.

Are the power switches ON at these units, are the fuses good, are the circuit breakers in the "on" position, and is the thermostat set correctly?

Check all of the electrical switches and controls that can turn electrical power off at the indoor air handler or at the outdoor compressor/condenser unit.

There are more of these switches than you might guess. Here's a list of what to check:

Main Electrical Power Switches for an Air Conditioner or Heat Pump

You will find service switches outside by the compressor, inside at the air handler, and you will also want to check the fuses or circuit breakers in the electrical panel.

• Check for a blown fuse locally
  
  inside the air handler, such as on or near a control board if switches are all on but there is no power being delivered to the indoor blower fan or outdoor compressor system.
  
  

Step 4: Check the electrical supply voltage

Even if electricity is turned on, if the supply voltage has fallen too far below the operating voltage range of your air conditioner it's likely that the system will not operate, particularly, you may note that the compressor motor won't start.

See VOLTS MEASUREMENT METHODS

Watch out: if you hear a humming noise at an electrical motor that won't start, turn the system off while waiting for repair. Shutting off the system might avoid costly damage to the equipment.

Watch out: if you are trying to run your air conditioner or heat pump from a backup electrical generator, it may be that while the generator supplies enough current (Amps) to run the equipment, it is not capable of handling the extra current draw (high amps) required to start the motor.

You may be able to circumvent this problem with an extra-boost or "hard-start" motor capacitor.

See CAPACITORS for HARD STARTING MOTORS

 

Step 5: Check the Indoor Air Handler / Blower Unit: Does it Start?

At the air handler 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.

• BLOWER DOOR SAFETY SWITCH - an open blower door will shut down the air handler unit
• MOTOR OVERLOAD RESET SWITCH - an overheated indoor blower fan motor could be keeping the blower unit fan from starting. If your blower motor will run after it has been shut down for a few hours (to cool off), we suspect this is the trouble.
  

  And if your blower fan is driven by a fan belt and an electric motor, of course check to see that the drive belt is in place and undamaged. If the blower fan belt is broken, the electric motor may run just fine (you may be able to hear it) but the blower fan assembly itself won't be turning.

  Details are at BLOWER FAN OPERATION & TESTING

  Similarly, the outdoor compressor/condenser unit also has its own cooling fan that could be off due to a problem with its fan motor, as does the compressor/condenser motor itself.
  
• CONDENSATE PAN SWITCH LOCKOUT - a condensate tray spillage detector switch (water in the overflow tray shuts down the air handler) will shut down the blower fan unit.
• Check (or your service tech will check) for a bad contactor or start relay for the Air Handler
  
  inside the air handler or outside at the compressor/condenser unit . These electro-mechanical switches are turned on by the unit's control circuit board and in turn they cause the blower motor or compressor motor (outdoors) to turn on or off as needed.
• Check (or your service tech will check) for a bad or failed starter capacitor
  
  for the fan motor in the blower assembly or outdoors at the compressor/condenser unit. This could also be leaving your system shut down, failing to start a blower motor fan indoors or outside the compressor condenser unit's fan, or compressor motor.

  If an electric motor hums but won't start or won't keep running, the problem could be a bad start/run capacitor.
  
  See CAPACITORS for HARD STARTING MOTORS
  
  Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start .

• 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.

Air Conditioner Won't Start or Stays Off Due to Condensate Pan Switch

Shown above is a FLOAT SWITCH on Condensate Tray that could cause a "condensate pan switch lockout" that prevents the blower unit from running.

Some condensate pans have their own separate overflow drain (a proper installation) or share their drain with the normal condensate drain (an improper installation).

This drip tray or drip pan, located under an air handler, is installed to catch air handler condensate leaks if the normal condensate drain system fails.

But on some air conditioning air handlers, the installer may provide a condensate overflow pan switch rather than a separate pan drain line.

Condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.

In this installation the switch is designed to turn off the air conditioning system if it finds condensate water in the overflow pan. The idea is to shut down the air conditioner before there is a more costly leak into the building insulation or ceiling.

Condensate Drain Failure

can also shut down the air conditioner.

In tracking down an air conditioner condensate leak, I found that the the condensate pump drain line, a small-diameter PVC pipe, was clogged with water mold.

That was why my air conditioner wasn't kicking on. It would of ended up costing at least a hundred dollars just for a service tech to fix something as simple as that. I had to take the pump apart and clean it. I took off the PVC drain line coming out of inside air conditioner and blow and clean it all out too. - Jacob Behrends, FL

The central a/c in my Florida home (in late July) would not turn on and the temp in the house was well above the preset temp of 78. Fearing a complete failure (the system is 17 years old) I googled my problem and eventually found this very clear and concise Inspectapedia site.

The past few days I could hear water dripping in the condensate drip pan and new something was up as this never happened before. I climbed my ladder at 11:30p and sure enough the condensate float switch was parallel to the tray meaning it tripped and turned the a/c system off.

Grabbed my shopvac and within minutes the immediate problem was solved - Now to clear the drain...Excellent DIY website! (July 24, 2014) Randy

So if your air conditioning system seems to be normal in all other respects but it simply won't turn on, check for a flooded condensate pan or a defective condensate switch.

See CONDENSATE PAN OVERFLOW SWITCH in our description of HVAC switches & controls

See CONDENSATE DRIP TRAY DEFECTS

 

Step 6: Check the Outdoor Compressor / Condenser Unit: Does it Start?

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.

Watch out: a refrigerant leak and low refrigerant can leave a compressor motor running so long that the motor overheats. An overheated A/C or heat pump motor may shut down on thermal overload, or it may simply burn-up and thus be destroyed.

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

1. COMPRESSOR START RELAY DOESN'T CLOSE
2. COMPRESSOR START RELAY CLOSES, COMPRESSOR DOESN'T RUN
3. CONDENSER FAN DOESN'T RUN

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 calling 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).

If you hear a buzzing or rapid clicking at the condenser unit (or at some indoor air handlers) the problem may be a chattering relay that is not pulling-in to turn on the unit.

First 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 to these 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 pressures in the system reach unsafe or abnormally 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 article on

the MOTOR OVERLOAD RESET SWITCH

Chattering relays, cause and diagnosis are explained in detail

at RELAY CONTROL SWITCHES

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

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).

See

• HARD STARTING COMPRESSOR MOTORS
• CAPACITORS for HARD STARTING MOTORS
• TEST a MOTOR START or RUN CAPACITOR

 

HVAC Condenser Unit Fan Not Working

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

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 include 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.

See

• LOCATE the STARTING CAPACITOR
• TEST a MOTOR START or RUN CAPACITOR
• ELECTRIC MOTOR DIAGNOSTIC GUIDE

Check (or your service tech will check) for a bad contactor or start relay

inside the air handler or outside at the compressor/condenser unit . These electro-mechanical switches are turned on by the unit's control circuit board and in turn they cause the blower motor or compressor motor (outdoors) to turn on or off as needed.

Check (or your service tech will check) for a bad or failed starter capacitor

for the fan motor in the blower assembly or outdoors at the compressor/condenser unit could also be leaving your system shut down, failing to start a blower motor fan indoors or outside the compressor condenser unit's fan, or compressor motor.

If an electric motor hums but won't start or won't keep running, the problem could be a bad start/run capacitor.

See CAPACITORS for HARD STARTING MOTORS

Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start .

...

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

AC fan motor slows, hums, won't run

Ac fan wouldn’t start could manually press contractors an it takes off a few hrs later can motor slows up an hums replaced motor an capacitor took right off when it shuts down it won’t start back unless pressing contractors then it takes off but eventually motor comes to a crawl could it be fan relay or control board it’s a trane xl 1500 xi - On 2021-08-28 1 by Fred deel -

Reply by inspectapedia.com.moderator (mod)

@Fred deel,

As you replaced the motor and capacitor, I'd look next for a bad fan bearing that's binding and loading the motor, maybe worse when it's run for a while and is heated up.

 

Rental Unit A/C keeps shutting off, won't re-start for hours

Hi! I hope someone can help. For almost 2 years the unit in my rental shuts down after running and will not start back up for hours. They keep sending someone out and it fixes nothing. They've tried everything and can't figure it out. I'm getting sick of it. It happens during heating and cooling. Please help. thank you. - On 2021-08-23 by Giamatteo -

Reply by inspectapedia.com.moderator (mod) - check first for thermal overload

@Giamatteo,

I agree that what you describe is really frustrating and annoying. But I've very little to go-on here.

When an electric motor shuts off and won't re-start for hours I suspect first that an electric motor is overheating and being shut off on thermal overload. When the motor is hot it can indeed take several hours for it to cool down, its internal overload switch to reset, and then for the motor to re-start.

Thermal overload can be caused by low voltage, bad wiring, a bad motor bearing, etc.

Our most complete list of things to check for the air conditioner not starting problem that you described are given on the page above, better than I could type it a new here.

Please take a look, if you haven't already done so. It sounds as if you need an experienced Heating and AC refrigeration service technician on site.

Have you tried a conversation with the local A/C service company service-manager to ask for a senior technician.

Would you say AC and furnace shutting off I'm a little confused.

Are you saying that in heating mode and then cooling mode your system shuts down and stays down for a long.?

And are you saying and heating mode that happens even when the outdoor compressor condenser unit isn't running?

In other words are you running on some other heat sources such as gas or oil?

So you can see that from afar, without a bit more detail we're not sure just what equipment we're discussing.

If I were to GUESS that by "furnace" you're referring to the indoor air handler that blows both cool air in cooling mode and warm air in heating mode,
AND
if I were to GUESS that you're asking about a heat pump system that provides both heating and cooling,
THEN
I would ask if your investigators included, in troubleshooting, a shut down caused by high head pressure at the compressor that in turn can be caused when a refrigerant metering device like a capillary tube or a TEV (Thermostatic Expansion Valve) freezes up or clogs with debris.

I really want to be helpful but beyond this we know so little about your installation that I worry about wasting your time sending you off in wrong direction. \

Outdoor compressor won't run in COOL mode

If I switched to the Heat mode on thermostat control, the fan blower to blow air inside the house works fine on Auto/on selections. When I switched to A/C mode, the fan blower NOT working on Auto and On selections, but the compressor Fan and everything seems working fine outside the house, The furnace control board status lights "RED" (Normal).

Please help ! - On 2020-09-09 by PETER -

Reply by (mod)

Still this sounds like a control board problem to me, but I'd start by reviewing the thermostat wiring;

it's also diagnostic to note whether this is a new problem or is one that's been present since installation

Sometimes my AC condenser unit will not turn on.

Sometimes my AC will not turn on. I have tried a number of things to troubleshoot this problem.

To be more specific my thermostat(s) will call for cooling, the (attic located) furnace blower will start but the condenser unit fan will not be running and the system will just circulate warm (ambient temperature) air.

The issue can be temporally resolved by removing power to the (attic) furnace and then connecting it again.

Unplugging the heater and plugging it back in. I typically accomplish this by flipping the furnace breaker in my electrical panel, though I have gone into the attic and simply unplugged the furnace from the electrical outlet just as a test.

After power cycling the furnace, the system will cool normally. That fix typically lasts about 24 hours before I have to power cycle the furnace again.

A couple years ago we had a problem with the system which turned out to be the Furnace Control Board. The technician that I called to come out fixed the board by reflowing the solder at one point on the board. A problem he had seen before. This fix worked. Thinking that this board was the source of my problems

I replaced the board with a new board from ICM Controls. This, unfortunately did not fix the problem.

After installing the new board everything was working normally, the test sequence worked as indicated, heating and cooling checked out when I set my stats for heat and then cool. After a period of time though, the same issue with the AC condenser fan not kicking on returned.

I inspected the electronics for the condensing unit yesterday and didn't see anything that stood out as a problem.

The capacitor is not bulging and everything appears to be connected properly. I even disconnected all the leads, applied dielectric grease to them and reconnected them. Still last night the problem returned. I tripped the breaker for the furnace and cooling resumed again.

I also have a zone controller in my attic. Two thermostats connect to the zone controller, zone controller connects to the furnace control board and drives two dampers in the ductwork to direct cooling to upstairs, downstairs or both. I see no signs of any issue with the zone controller.

I checked the LEDs on the zone controller while the AC was not working properly and found no fault or error indications. The zone controller is powered from a transformer inside the furnace so when power cycling the furnace, the zone controller is power cycled as well. I have not yet tried bypassing the zone controller. - On 2020-08-18 by Todd B. -

Reply by (mod)

Todd:

Have you done a detailed diagnostic of just where power is present or not?

At the control switch, at the board, at relays, etc.?

What about a slow-draining condensate pan and a condensate pan switch that shuts off the unit?

And incidentally, you can have a failed start capacitor that isn't bulged. It's pretty easy and inexpensive to try swapping in a new one.

...

Continue reading at BURNED-OUT COMPRESSOR or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AIR CONDITIONER WON'T START FAQs diagnostic questions & answers posted originally at this page.

Or see these

Recommended Articles

• BLOWER FAN OPERATION & TESTING
• CAPACITORS for HARD STARTING MOTORS
• COMPRESSOR / CONDENSER DIAGNOSTICS
• CONTROLS & SWITCHES, A/C - HEAT PUMP
• DIAGNOSTIC GUIDE A/C or HEAT PUMP
  • AIR CONDITIONER WON'T START
  • BURNED-OUT COMPRESSOR
  • CAPACITORS for HARD STARTING MOTORS
  • CONDENSATE PAN OVERFLOW SWITCH
  • CONDENSER FAN WON'T START
  • ELECTRIC MOTOR DIAGNOSTIC GUIDE
  • FURNACE FAN WONT START
• ELECTRIC MOTOR DIAGNOSTIC GUIDE
• HARD STARTING COMPRESSOR MOTORS
• MOTOR OVERLOAD RESET SWITCH

Suggested citation for this web page

AIR CONDITIONER WON'T START at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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