Air conditioner or heat pump won't start:

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 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 temperature, or an air conditioner that won't start.

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

Air Conditioner Won't Start - Air conditioning system is not running at all:

Is there no cool air at all coming out of the supply registers? Is the air conditioner or heat pump indoor or outdoor unit silent? If so it's not running. Here is what to check first.

Check first 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.

Here are the details of what to check in what order if your air condtioner or heat pump doesn't start at all when you set the room thermostat to call for cooling:

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. 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, check that when you set the thermostat temperature down at least 5 degrees below room temperature the thermostat calls for cooling. If it doesn't then check for broken or shorted thermostat wires anywhere between the wall thermostat and the control board at the air handler.
   
   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.
   
   If the thermostat is working but the compressor condenser unit won't start, you could skip ahead
   
   to COMPRESSOR / CONDENSER DIAGNOSTICS but I wish you'd double check the remaining steps in this article first because there are some sneaky snafus listed below that might still be the problem.
3. Check that electricity is on for the equipment.  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:
   
   Electrical power switches and service switches outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel. Don't forget to check that the access covers to the equipment are properly closed and latched. Otherwise a
   
   BLOWER DOOR SAFETY SWITCH could be keeping the equipment from running.
   
   There are several other safety 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.
   
   Some hard-to-find electrical switches on an air conditioner or heat pump could be keeping your air conditioner from starting, such as
   
   a FLOAT SWITCH on Condensate Tray that could
   be
   
   causing CONDENSATE PAN SWITCH LOCKOUT - condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.
   or
   
   a blower MOTOR OVERLOAD RESET SWITCH could be keeping a fan motor from starting.
   
   A bad or failed starter capacitor could also be leaving your system shut down, failing to start a blower, fan, or compressor motor.
   
   See CAPACITORS for HARD STARTING MOTORS
   
   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.
4. Check the electrical supply voltage. Even if electricity is 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, particlarly, 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. We gave the link to CAPACITORS for HARD STARTING MOTORS just above.

If Steps 1-4 All Check Out OK but the A/C System Is Not Working Check These Items

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 check the following:

Thermostat problems: Air Conditioner Thermostats:

Air conditioner won't turn on, or fan won't turn on or won't turn off.

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

Air Conditioner Air Handler Blower motor Won't Start?

See MOTOR OVERLOAD RESET SWITCH just to be sure that the air handler blower fan motor has not shut off on thermal overload.

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 un-damaged. If the blower fan belt is broken the electric motor will 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

Air Conditioner Won't Start - electrical problems

If the air handler or indoor blower assembly does not start in response to a call for cooling the no-start problem is probably at the indoor thermostat or at the blower assembly itself.

If the indoor air handler blower runs but the outdoor compressor/condenser unit never starts then the problem is more likely there. If the indoor air handler runs but the outdoor compressor/condenser does not,

see COMPRESSOR / CONDENSER DIAGNOSTICS

1. Check that electricity is on for the equipment, everywhere. 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.

See KEY HVAC SWITCHES service switches, circuit breakers, fuses, power

2. Check all of the safety and interlock switches: 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

• BLOWER DOOR SAFETY SWITCH - an open blower door will shut down the air handler unit
• CONDENSATE PAN SWITCH LOCKOUT - a condensate tray spillage detector switch (water in the overflow tray shuts down the air handler) Below in this article we give more details about condensate drip tray or overflow tray safety switch problems.
• RESET BUTTON, ELECTRIC MOTOR - electric motor overload or overheat switches - a motor overload switch will keep fan motors from running

See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system both at the outdoor compressor/condenser unit and indoors at the air handler and duct system.

Electrical problems: air conditioning system won't start: it may sound silly, but is the air conditioner turned on?

Has the cooling thermostat been set to "cool" and the temperature set below ambient room temperature?

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? Photo 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?

• See A/C - HEAT PUMP CONTROLS & SWITCHES
• See THERMOSTATS for more diagnostic tips.
• Check the electrical power supply: is power on to all of the A/C equipment and is voltage within normal operating levels for the equipment. We cited this and included some warnings earlier in the article above.

Air conditioner compressor problems, including compressor noises, hard starting, and burned-out compressors, are explained in detail beginning

at COMPRESSOR & CONDENSING COIL and including topics such as

• BURNED-OUT COMPRESSOR
• HARD STARTING COMPRESSOR MOTORS
• MOTOR OVERLOAD RESET SWITCH
• ELECTRIC MOTOR DIAGNOSTIC GUIDE
  
  
• Check for a blown fuse locally inside the air handler, such as on or near a control board.

• Check for a bad contactor or start relay inside the air handler or outside at the compressor/condenser unit

• 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.

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 - we give links to those diagnostics in step 2 above.

See CAPACITORS for HARD STARTING MOTORS

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

Condensate pan switch lockout: an attic or other air conditioner air handler condensate 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.

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

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

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 leaks into the condensate pan can 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

(July 24, 2014) Randy said:

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!

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

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-08-28 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.

On 2021-08-28 1 by Fred deel

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 whe 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 2021-08-26 by inspectapedia.com.moderator (mod)

@Giamatteo,

You're correct, it looks as if I haven't correctly understood your comments and question.

I apologize.

Let me take another look at think about it a bit.

Keep in mind that I have only a microscopic bit of information compared to the people who've been to your site. I can't be smarter than they are.

...

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.

On 2021-08-26 by Giamatteo

inspectapedia.com.moderato not sure what made you assume I haven't contacted someone or read the post prior to commenting. 2 different well known companies have been out. Even the owner of 1.. numerous times now. I also reviewed the page before I wrote the comment. I didn't see anything pertaining to the A/C and furnace both having the same issue in shutting off randomly and not starting for 12 or more hours. To me that wouldn't be "short" cycling. No errors on the circuit boards eirher. Thanks anyhow.

On 2021-08-23 by inspectapedia.com.moderator (mod)

@Giamatteo,

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

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.

On 2021-08-23 by Giamatteo

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 2020-09-24 by (mod)

Let's start by checking the diagnostic steps given on the page above, Dorie.

On 2020-09-24 by Dorie

My heat and central air suddenly stopped today. My thermostat clicks but nothing happens. Hmm.
Any ideas?

On 2020-09-09 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

On 2020-09-09 by PETER

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 2020-08-18 by (mod) - Sometimes my AC condenser unit will not turn on.

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.

On 2020-08-18 by Todd B.

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.

...

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

• 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
• BLOWER FAN OPERATION & TESTING
• COMPRESSOR / CONDENSER DIAGNOSTICS
• CONTROLS & SWITCHES, A/C - HEAT PUMP
• ELECTRIC MOTOR DIAGNOSTIC GUIDE

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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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