Air Conditioner / Heat Pump Won't Start - Diagnostic FAQs

Diagnostic questions & answers

Switches that Can Turn off the A/C or Heat Pump Unit FAQs

Remember to check first for the obvious:

• power switches to the indoor and outdoor equipment
• safety switches on blower compartment doors
• thermostat setting that is not calling for heating or cooling

Clogged condensate drain turns off Heat Pump / Air Conditioner; do I need a new pan?

My Carrier heat pump A/C has stopped functioning. The repairman says that there is a condensate collection pan as a part of the indoor unit in the attic of the house and that a drainage line is plugged, and the rising water level in the pan actuates a switch shutting off the fan/compressor unit situated outside the house at grade level.

He says that the pan unit needs to be replaced at cost of about $400. Is this diagnosis credible? Any alternate diagnosis and remedies? On 2015-07-01 by NCK Bethesda Md

Reply by InternetLizard

+NCK Bethesda Md Can you try to find the drainage line and either clean or replace it? Sometimes there's also a pump that should be checked.

Reply by (mod)

It should not be necessary to replace an entire condensate drain pan simply to un-clog a clogged drain line. I'd want more explanation from that service tech before proceeding.

See CONDENSATE PAN SWITCH KEEPS AIR HANDLER OFF? - this or a door switch can keep the air handler "off"

 

High head pressure turns off our Condenser Unit

I had to replce both condenser fan motors because the bearings where to tight to spin freely.

What is happening now is that the unit is running for maybe 4 or 5 hours than both the first stage and the second stage compressers are tripping on high head pressure. What would you suggest to try. On 2012-07-18 by Terry

Reply by (mod) -

Terry the problem you describe is one I'd expect to turn over to a trained service tech - as I'm not sure myself what's happening. Often what's reported with high head pressure is a thermostatic expansion valve that is stuck shut or dirt clogged or frozen up.

Intermittent A/C or Heat Pump Won't Start FAQs

A/C worked just intermittently; Put in new system; same problem.

A/C works intermittently. Will work fine for a few days and then just start blowing uncooled air.

When heat pump blows uncooled air it begins making metal on metal loud sounds. If I shut it off for a few hours and turn back on it works fine. Have had 4 visits from technician to determine problem.

Was told I needed new heat pump. So we put new one in. Same problem is still occurring. A/c running intermittently and heat pump making metal on metal sound when air is no longer being cooled.

They have checked pressure, air handler , freon, circuits, etc. please help me. This has been going on for 3 weeks now. It is 95 degrees currently. On 2016-06-06 by Paula

Reply by (mod) -

It's not clear to me why the need for a new heat pump would result in an intermittent problem, Paula.

Seems to me if the heat pump - presumably the compressor mortor - has failed, it would fail consistently.

More likely there is a problem with a

• control board
• control relay,
• wiring connection
• supply voltage
• a fan or compressor motor that is having trouble starting or has a bad start capacitor
• something else I've forgotten to list

Call your HVAC service company manager, calmly discuss the problem with him/her, and ask for help from one of their senior, experienced techs. Keep us posted.

 

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 both 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. \

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.

 

Why won't the heat pump re-start and cool after it has run and then turned off?

My daughter has a mobile home that is 30 yrs old. The heat pump is used for both air conditioning and heating.

When the breaker is turned on, the air conditioning unit will run until it has reached the tempature set on the new thermostat. It will then turn off.

It will not restart when temp rises and outdoor section runs but no air is flowing through ducts.

What is causing this from not restarting and cooling properly? On 2016-09-13 by Doug Trombley

by (mod) -

Doug the problem is likely to be a control, a start capacitor, or the compressor motor itself, not the main breaker powering the unit.

See AIR CONDITIONER WON'T START - topic home

Heat Pump Cools down to Thermostat Set Temp but Compressor Won't Turn Back On

Trane Xl1600 heat pump cools down to set thermostat setting and then will not turn compressor back on and blow hot air in the home.

This seems to happen when Phoenix temperatures reach 100 degrees. Replaced both start and run capacitors a couple months ago as issue started last summer was working fine until 100 degrees. Any assistance with this would be greatly appreciated. On 2015-06-16 by Larry Trane

by (mod) -

Larry if replacing the start/run caps didn't fix the trouble I suspect either a failing control board / relay or a compressor motor that is failing beyond the ability of the starting cap to kick it off. Perhaps your tech will check the current draw of the various motors involved to see if that's going on.

by Larry - the capacitor is leaking oil

Noticed my run capacitor is leaking oil?

On 2015-06-16 by (mod) -

Then it's shot and should be replaced.

Unit Runs in Heat or Cool but Not Both Modes FAQs

System runs in HEAT but 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.

Consider that if your system is a heat pump, even though your indoor unit seems to run in HEAT mode it may be that it's running in backup heat moder and not requiring the outdoor compressor/condenser unit.

Heat pump won't start or run in A/C mode but runs in HEAT mode

My heat pump will not run in A/C mode, but will run in heat mode. The inside blower will run. I swapped out the themostat from downstairs (same type-mercury swithches), still no A/C. Is there a fuse that may have blown? (Aug 6, 2015) Gary

Turned on unit and nothing happens. Checked fuses, connections,wires.Don't know what the problem is so don't know if i could fix it myself or need to call someone. Senior, on fixed income. Help. (Aug 15, 2015) Ella

Reply:

Gary:

I suspect that the compressor/condenser is not starting and that in heat mode you're running on backup heat. If the motor never starts at all and if there is power to the unit, the problem is likely to be in a thermostat setting, a start relay or control board.

See DIAGNOSTIC GUIDE A/C or HEAT PUMP or

select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Heat pump worked in Air Conditioning mode but not in HEAT mode

My Bryant heat pump worked great all summer,

i switch it over to heat it came on for 1 second ,now it wont do any thing on heat or air ? -Wayne Bostian

Moderator reply:

Wayne

Let's start with the basics: has a breaker tripped or fuse blown? Is there power to the unit?

What came on for a second: the air handler indoors or the compressor/condenser unit outdoors?

Heat Pump Works Only in Cooling Mode, it Won't Heat

My heat pump runs when calling for cooling, but when calling for heat it won't run ,the Aux comes on On 2015-09-25 17:16:19.990602 by George

Reply by (mod) -

George,

I suspect that either the outdoor temperature is below the cut-in temperature for your heat pump or there is a bad control board, relay, or temperature sensor.

A/C or Heat Pump Won't Start, Contactor Relay FAQs

Air Conditioner or Heat Pump won't start: Contactors won't pull in when they should

New package system a/s 10 ton first stage of cooling works perfectly when both stages of cooling are called for contactors will noy pull completly in .Yes good ground,208 330 yes 24 volts at transformer .Trane has no clue.

A/c contractors have no clue .

Installed new honeywell t stat in side pkg unit at factory wiring .Yes changed transformer yes have changed both contactors yed there is a matching unit next to this one both wired same other unit runs perfectly .Im at a total loss 30yrs exper s.O.S. Mp#3 954 -803- 1646 - On 2012-07-26 by Anonymous

If a Contactor is Not Pulling-In when it Should, Measured the voltage at the contactor

Have you measured the voltage at the contactor?

(Not at the transformer) With the contactor not pulling all the way in, it sounds like you don't have 24 VAC at the contactor.

If this
is the case,

check the connections between the transformer and contactor for a resistance
joint. Could be a wire-nut connection or the t-stat resistance (either via a relay or a
collector feed). - On 2012-08-03 by Coastal - H

Reply by (mod) - a bad connector or on occasion a partial short or leak in the low voltage wiring could drop the voltage at the contactor.

Coastal, thanks for that excellent suggestion made to Anon. It makes sense to me too that a bad connector or on occasion a partial short or leak in the low voltage wiring could drop the voltage at the contactor.

More diagnostics are

at COMPRESSOR START RELAY DOESN'T CLOSE

Daniel

 

Air Handler Blower Won't Start FAQs

A/C blower fan starts only if I spin it by hand

Reply:

Anon: look for and replace the fan motor start/run capacitor. - (June 4, 2015) Anonymous

Furnace blower fan won't start

I have a oil hot air furnace,

the gun starts but blower won't come on,

replaced blower control panel,need some advice,

Reply:

Anon if by gun you mean the oil burner and by blower you mean the air handler blower that moves building air through the ductwork those are of course different topics.

If the blower never comes on the limit switch on the furnace should turn OFF the oil burner.

If it doesn't SHUT DOWN the system immediately.

CHeck blower motor wiring and blower motor itself, as well as controlling relays.

 

indoor air conditioning unit has no power and indoor and outdoor AC units won't start

Reply:

Look for an open wire in the circuit or a blown fuse.

Outside unit runs, inside unit won't turn on except in manual FAN ON

My outside ac unit will turn on, but the fan inside will not blow air through the vents.

But if I turn the ac to the off position, and the fan to "on" instead of auto, the fan will start blowing air.

It is like I can either use the fan or use the ac unit, but not both of them On 2011-08-15 by clexow

by (mod) - Turning on the blower manually keeps it running

When you turn off your A/C and switch the fan to "ON" you are manually turning on the indoor blower assembly fan itself, independent of the A/C system.

On the thermostat if you set the appropriate switch to "COOL" that turns on your A/C system and the fan will run automatically under control of the thermostat and additional control circuits in the air handler and in concert with the outdoor compressor/condenser unit. Normally if a thermostat has the necessary switches and it's wired correctly, you could set the TT for COOL and ALSO set the fan either to AUTO or to ON. in "ON" position that forces the fan to run continuously regardless of whether or not the TT is calling for cooling.

But you're not seeing that.

When you turn on the A/C at the TT but you are not getting cooling, either a service or control switch is OFF at your equipment (or in the main electric panel) or a thermal overload reset switch has turned off the equipment, or a motor such as a compressor motor has seized.

I'd start by going through the LOST COOLING CAPACITY diagnostic steps to be sure everything is turned 'on', and if that doesn't help you probably need an HVAC tech service call.

Question: there was a humming sound, with a high pitch squealing noise - Air Handler Failure?

I had new condenser installed about a month ago, due to it tripping and a/c stops working.

A couple days later after install, there was a humming sound, with a high pitch squealing noise that's not so loud.

I could hear condenser from inside the house start up loud. This noise lasts for 5-10 minutes and it regulates to the normal sound. Air coming out of house's vents is not cold.

So no a/c inside the house. With the next start up of the a/c, the cool air blows again.

Tech keeps saying it's not the condenser and I need to replace everything up in the attic.

I have warranty with the new unit but tech is saying the unit is fine. Can you please help? Thanks. 2017-08-04 by MP

Reply: humming suggests failing motor or bad bearing

I'm a bit worried, too, that there is a motor or bearing or pulley failing in the air handler in your attic.

The humming is often a motor having trouble starting. We can't do more arm-waving here: you need the tech to take a careful look in the air handler to track down the noise and report to you its source.

Watch out: diagnose the problem more accurately before even considering replaing "everything in the attic" - it would be shameful to sell you a multi-thousand new attic air handler when all that was really needed was a fan motor or pulley or belt.

Should I turn off the unit at the thermostat if the air handler blower has failed?

if the blower fan motor of my central air conditioner unit stop working should i turn off the unit from the thermostat and for the air handler as well. 2017-05-23 by Anonymous

Reply:

Anon

Turning off the air handler at the thermostat may make some sense as running the air handler when the compressor/condenser isn't working won't provide cooling.

You can move the thermostat A/C control to OFF, or you could accomplish the same thing by simply setting the thermostat temperature well above room temperature.

Alternatively you could turn the cooling OFF but if your thermostat (or air handler limit control) has a FAN ON or MAN switch position you could continue to circulate indoor air as well as filter it - that might provide a little comfort even when the cooling system is not working.

Haier Thermocool air conditioner won't run, green lights blink

Reply: contact information for Haier Thermocool air conditioners

Kinsey you don't give your location. As the diagnostic lights on your Haier Thermocool are not helping you I'd contact the company to ask their advice. If you're in NIgeria as I think from un published part of your comment, see

• HPZ Nigeria LTD. 45-47 Town Planning Way, Ilupeju Industrial Estate, Ilupeju Lagos Nigeria Thermocool customer service: 0 700 843 766 2665

A/C or Heat Pump Outdoor Compressor/Condenser Unit Won't Turn On FAQs

A/C compressor unit won't run after a power outage

My power went out the night before and now my outside compressor/condenser will not turn on.

The unit turns on in the inside but it just blows hot air because of compressor/condenser will not turn on. What can be the problem?

Reply:

Check that you actually have power to all of the equipment; look for a blown fuse, tripped breaker.

A/C Concern said: problem was with the power supply from the electric utility

Please disregard my last post. Something happened to our electricity and it was not producing enough electric to run the A/C. But now electricity is fine and A/C is working.

Lifting the condenser unit caused it to fail?

We lifted our compressor fan and placed blocks underneath so we could pour a concrete slab and now it won't kick on but the indoor unit is working. What do I look for? 2016-06-26 by Thedevotifamily

Reply:

Simply elevating a compressor/condenser up a bit should not in itself cause it to fail to turn on.

Look for disturbed electrical wiring such as a broken wire, a switch that was left OFF, or a thermostat wire or control wire that was broken or not re-connected.

 

Outside condenser (and its fan) won't come on after coils were cleaned

Outside unit fan will not come on after my husband cleaned the outside unit coils.

He pulled the outside switch to cut the power but he also said that the compressor and fan were off before he pulled the outside switch. Help, he's an operator not an a/c man. 2015-11-06 by Anonymous

Reply:

It sounds as if a switch remains off or is not functioning but of course there could be a different issue such as water-damaged wiring or controls.

I found this diagnostic article by searching InspectAPedia for "air conditioner won't start"

AIR CONDITIONER WON'T START  where you'll find ten easy diagnostic steps to try.

Rheem RANL-037JAZ won't start

Our A/C (Rheem RANL-037JAZ) was installed on 2011, each year we have less than 20 days to use it because it's not that hot here, but some times it worked fine, and more than half of the time it won't start, it made a clicking noise when I turned it on, and the fan spined a few turns then quieted down, it will try to turn on after a while.

The inside air blower always worked. I thought maybe it was the line voltage is too low, so I bought a Supco's SPP6E booster capacitor put in parallel with the run capacitor 43-101665-48, but it still didn't work.

Reply:

Indeed a bad start/ run capacitor will prevent the system from starting. Typically you'll hear a start relay click and the compressor motor hums but doesn't run. LEAVE THE SYSTEM OFF until it's repaired lest you damage the equipment.

 

Indoor air handler runs but outside compressor is not running

A/C not cooling; however, air is blowing through the ducts. Outside unit fan is not turning, but noise is coming from the unit. System is 20 years old. Based on what I read above sounds like it COULD be blower motor at outside unit or the capacitor.

Any other words of of wisdom. If blower fan or capacitor, are these expensive repairs?? Thanks for your valued input. On 2011-08-15 by DON RAUTH

by (mod) -

If your outside compressor/condenser is not running it could be a bad capacitor (not a big repair) or a bad compressor (big repair). But the fact that the fan is not turning but you hear noise means you need to start with checking the fan motor and its start/run capacitor.

Watch out: when a condenser/compressor unit fan stops running, leaving the system ON and the compressor running can cause the compressor to overheat, experience high head pressures and thus lead to compressor damage. Turn off the system until it's diagnosed and fixed.

Blower runs but no cool air is circulating - do I need a new A/C system?

My AC system stopped working this evening. The blower was still working but no cool air circulating.

I checked the outside system compressor/housing and found ice build up on the tube running from the compressor to the house.

The unit is almost 14 years old, and we've had nothing but 90+ temps for the past two months

. Is it just time to purchase a new outside compressor unit and a new inside AC coil unit. My neighbor just replaced his at a cost of $7,700. On 2011-08-09 by Bill in VA

by (mod) - check & diagnose the problem before replacing an air conditioner

Bill, before springing for a completely new air conditioning system, I'd ask for some diagnostics on the present one. Your old system might need simply a control replacement or a refrigerant leak repaired.