A/C & Heat Pump Outside Compressor/Condenser Won't Start FAQs

Condenser unit won't run

Compressor/Condenser Unit Won't Start / Won't Run FAQs

Helpful diagnostic articles:

• AIR CONDITIONER WON'T START
• A/C - HEAT PUMP CONTROLS & SWITCHES - Is it turned on?
• BURNED-OUT COMPRESSOR
• CONTACTOR RELAY DIAGNOSIS & REPAIR
• CONTROL CIRCUIT BOARD, A/C

Outdoor Compressor won't start

I have a carrier condensing unit that the compressor wont start. I have voltage at the contactor and have replaced the start cap and run capacitor.

Odd to say, I'm drawing 11 amps on start and run wires coming from the contactor but I hear nothing from the compressor! Any suggestions? (Aug 2, 2014) Lou

I have a Carrier heat pump that i can't get the blower to come on. I have replaced the board and thermostat and not sure what to do next. Any suggestions? (July 13, 2014) Anonymous

Reply:

Lou and Anon:

Check for and confirm voltage right at the wires to the compressor motor;

Check for a bad start relay switch or contactor.

Then see our more-detailed diagnostic steps at AIR CONDITIONER WON'T START

What keeps a heat pump from kicking on

What keeps a heat pump from kicking on? by Steven Hagewood

by (mod) - What keeps a heat pump from kicking on

That sounds like a control board or switch problem.

• Check first for power at the unit
• Then check the thermostat settings
• Then check the control boards/relays
  
  Even a loose wire or corroded space connector can be at fault or there can be a more costly issue like a failed compressor motor.

by Steven- cleaning fixed the control

I cleaned it and it works fine now

Outside Compressor Motor Won't Run

The outside unit fan runs fine but compressor motor itself does not start - Tim

by (mod) - outside fan runs good but compressor does not

Tim, as your A/C compressor is not running you need some diagnosis and repair. It might be something minor like a starting capacitor, or a larger problem such as a burned out compressor motor.

See these diagnostic procedures

• AIR CONDITIONER WON'T START

AC Won't Cool - outside unit won't come on

My AC wont cool. I have checked the filter and is clean. The air handler is clear. The air handler stays on and the out side unit will not come on the unit is 2 years old

I cut the power to switches and reset them. The out side unit came on for the night and now will not turn on. I push in the relay on the out side unit and he house starts to cool but when I let go it shuts off again. On 2011-08-27 by Peter

Reply by (mod) - hard starting compressor

Peter:

sounds like a hard starting compressor; ask the hvac tech about a start/run capacitor kit.

Delay in when the compressor starts and cool air begins

I'm no HVAC expert but i understand how it all works. My problem is that the indoor blower comes on, the outdoor fan comes on, but the compressor seems to take a while to come on, sometimes up to 10 minutes, meanwhile its blowing non cold air around the house.

I go outside next to the compressor, when it comes on i hear the fan but not the compressor (no buzzing sound), but it does come on later and then the freon pipe gets cold and I'm all good.

I am in Miami and it is getting pretty hot here this summer, is this delay normal ? or is it a prelude to a more major issue ?

Thanks. 2018-08-08 by Paul

Reply: normal delay in outdoor compressor motor start-up?

Paul,

The delay you described could be normal. For example in my office where we use a Sanyo split system air conditioner, the indoor blower unit may keep running at low speed but the outdoor compressor will turn on and off depending on when the control board (with its own computer) figures that it's needed. That's an energy-saving feature.

But IF your AC blower is delivering warm air AND the room temperature is still above the thermostat setting, then yeah something's wrong, perhaps with a sensor or control relay on the unit. That may not be a major repair cost ( a new compressor would be a major cost).

IF you hear the compressor humming but not starting, that's not very good and means there's a more serious problem. Call your service tech. Let me know what you're told.

Compressor won't start until it has cooled down

When my outdoor compressor/condenser unit is cold it will start and run.

But after an hour the cooling fan stops but the compressor continues for a short time then quits.

It will continue to start but will not start and run until it cools down. Any ideas? Wayne

Reply:

Look for an overheating fan motor, relay or control board or possibly a refrigerant metering valve that is sticking

See

COMPRESSOR / CONDENSER COOL with AIR or WATER? FAQs - spray or immerse in water as a diagnostic or to improve efficiency

A/C compressor not turning on

The outside unit of the heat pump is icing up on the compressor and the compressor won't start. - Steve 8/7/11

In the indoor air handler the fan in my house goes on but my outdoor compressor unit does not. All the electric seems to be fine . Could it be my thermostat? - Sam 8/7/11

the accu very heat,although it is new gen. clean,2Tr LS-60psi & HS-100psi, - Anon 8/6/11

Reply:

Steve: icing at the compressor/condenser sounds like a refrigerant charger or refrigerant metering problem.

Sam: if your thermostat turns on the air handler but the compressor/condenser does not run, the problem is not the thermostat. Check out the diagnostic suggestiosn beginning at LOST COOLING CAPACITY (link at the ARTICLE INDEX the bottom of this article )

Anon: those pressures look kind of low, no? What does your HVAC tech say about normal for that system?

Condenser unit won't run after I cleaned off wet leaves

I took the top off my central air unit to wipe the wet leaves, & dirt out i put the top back on

i tried to be careful & now it has power but fan doesnt turn did i maybe bump a wire plz help me its so hot i dont h $ right now is there a reset button On 2017-06-13 by Anonymous

by (mod) - bumped a plug-type wiring connector and loosened it.

Anon

Most-likely you bumped a plug-type wiring connector and loosened it.

With power OFF to the unit, you can remove the cover and look for wires to the fan motor - follow those to see if there's a disconnection.

Assuming that the fan and condenser/compressor unit was working last time it was used, and that you had power off to do your cleanup, it's not likely that there is a fan reset button problem; the reset on a fan motor trips if a motor overheats; not all fans have such a button as some thermal overload resets are internal and will re-set themselves automagically after the motor cools down.

A/C Compressor/Condenser Won't Start

The unit won't start at all. I get 236.4 VAC coming in on the line, but the relay won't latch. So, I went and bought a new relay and replaced with not luck.

I then used the manual push button on the relay to latch the connection and heard a light hum.

I then gave the fan blade a little help and it started turrning. However, it did not get up to speed and the compressor never kicked on. The relay won't stay latched regardless. Any ideas for me here? On 2012-07-17 by Chris Bauer

by Bobcat

Chris B. Is the condensing unit you refering to, a heat pump or straight AC?

Is "light humming" coming from the condenser motor or fan motor?

When a fan will run after you give it a spin I suspect a bad fan start capacitor, but if the motor never gets up to speed it could be a bad run capacitor or a bad motor itself.

So I suspect a bad start capacitor OR a failed motor.

To rule out a problem at the thermostat itself need to measure voltage at the thermostat wires that attach inside the outdoor unit. On 2012-07-18

by Chris B.

This is a straight AC unit. Yeah, I should have checked the 24 volt. I will do that. I appreciate any help I can get here.

This is straight AC with control that runs to the furnace

Outdoor Condenser Unit Won't Start after I damaged a wire

I clipped the thermostat wire to my central AC unit outdoors, and the power to both the main blower in the attic and outdoor AC unit will no longer start.

I spliced the thermostat wire back together and replaced the contactor on the outdoor AC unit in hope that it would fix the problem.

However it did not.

I have tested for a current on the contactor in outdoor AC unit and it's there, but there is no current in the thermostat wire leading up my house and into the attic. When I test for current in the thermostats located throughout the house there is also no current. What would be the next move?

I plan on going into the attic where the blower is and replacing the entire thermostat wire leading to the outside AC unit and checking for a burnt fuse in the circuit.

There wasn't a reset button on either appliance that I could find, only a transformer switch outdoors near the AC unit and a "AC on / off" switch in the attic. Any suggestions from this point would be appreciated. Thanks On 2012-06-21 Josh Massih

by (mod) -

Josh, usually the thermostat is powered by a low-voltage transformer - you need to find it and start tracing wiring there to find where the system has either lost power or has a broken or shorted wire.

And of course, if power is off to your equipment, then power is probably off to the transformer too.

by Josh

So, by following and replacing the thermostat wire from the outdoor ac unit into the attic blower, I will find a low voltage transformer that powers the thermostats throughout my house, correct? What would the low voltage transformer look like? Could it be a fuse like I would find in a car?

by (mod) -

Josh, I suspect you'd be better served calling an expericenced hvac tech. The low voltage transformer is visible in some of our fan limit photos found at FURNACE CONTROLS & SWITCHES - (search InspectAPedia for that title)

Question: outside air conditioner compressor made a loud noise, then smoked, now it seems to be working

My compressor made a loud noice,then puff of smoke came out.I turned it off,when I turned it on again it worked and is heating.any known reason for this? - (Jan 4, 2015) linda cape

Reply:

Linda I dont know what happens but it doesn't sound good. If it was really smoke -as in something burning, I'd leave the unit off and ask for a service call.

Possibly it was not true smoke but a refrigerant leak which might, depending on leak rate and weather, might appear as white "smoke'. - Also a need for a service call.

it's also possible your system is running but on backup heat.

DO let us us know what your heating service tech finds as it will assist other readers. And do not avoid a service call since if there was a leak or burn-up the system may be unsafe.

Question: I filled up my air conditioner with gas but it's not working: now what?

So what is the solution of this problem as I have already filled gas recently. Thanks !! - Anon 9/16/11

Reply:

You need a service call from an experienced diagnostician. We don't have enough information about your case to specify a fix.

Or take a look at

• LOST COOLING CAPACITY

for more diagnostic guides

Question: My air handler works but the outside AC unit doesn't come on all the time - bad compressor?

my air handler works, but the outside ac unit does not come on all the time.

It's intermintent. My HVAC guy did pressures and said I have a bad comprresser (20 year old unit) and I'm better off getting a new ac unit. Sometimes it will come on and stay on. Other times it will not come on at all. - JPI 6/16/12

Reply:

JPI, I'm in no position to second guess the expert who was on the scene - s/he sees clues and performs test way beyond what we can do in a text Q&A. But here are some thoughts:

a failing compressor motor can become increasingly hard to start; the tech can install a hard-start capacitor to get the motor running again - for a time - and it might be worth a try: that's an inexpensive part and usually fairly straight-forward to install.

Question: Our new AC won't keep running

uhm my A/C compressor broke about two weeks ago, we got a new one but when the fan starts the compressor works for a while and then it shuts down, and its turning on and off and I dont know whats wrong - Help. 7/25/11

Reply:

Help: if your new A/c won't keep running after it has just been installed, surely you should call the installer and expect it to be repaired under warranty - it's just been two weeks.

Even if all of the new equipment is in good condition, an example of a problem that could occur during compressor replacement is that contamination, moisture, dirt in the refrigeration system (which is made worse if a compressor burns up) can interfere with proper system operation.

Question: My AC won't work if I set the temperature down below 25C

my a/c is working properly when it is adjusted at 25 degree temprature, but if it is lower than 25, the compressor is disconnected by the overload protection. what is the problem with my a/c. - Mohamed 8/5/11

Reply:

Mohamed an A/C compressor may cut off on thermal overload due to overheating from local conditions around the compressor/condenser unit (no shade, very hot weather), an older motor that is failing, a start capacitor problem, or even a refrigerant metering device problem.

If there's nothing obvious to you, I'd get a service tech to take a look. Keep us posted.

Condenser unit fan runs but the compressor won't start and the contactor is welded closed

Condenser fan works but compressor will not start and contactor contacts are stuck closed what are the steps to solve - On 2017-07-31 by Jim

Reply by (mod) -

Jim

Watch out for death by electrocution - if you're not trained to fool around inside the equipment it's rather dangerous.

That said, your tech would probably start by measuring the current draws - if it's abnormally high the motor may be seized.

Else she'll typically try the inexpensive shotgun repair approach: change out the start/run capacitor.

Heat Pump Won't Start on Hot Days and in Heat Mode Also won't keep running

This summer my A/C would not cool on very hot days.

In "heat mode" the unit starts, runs for 5-10 seconds and then stops - 2018-11-05 by Vickie -

by (mod) - my heat pump short cycles on and off when in heating mode

Vickie

I am sorry that I can't offer a specific diagnosis but frankly we just have too little information to say anything useful beyond the compressor failure articles listed right at the end of the article above.

I would call an HVAC service technician for on-site diagnosis and repair help.

But before doing that, also take a look at our description of the common causes of air conditioner or heat pump "short-cycling" on and off, found at

SHORT CYCLING AC COMPRESSOR

Question: Carrier AC Compressor won't start

I have a Carrier heat pump that i cant get the blower to come on. I have replaced the board and thermostat and not sure what to do next. Any suggestions? Anonymous

I have a Carrier condensing unit that the compressor wont start.

I have voltage at the contactor and have replaced the start cap and run cap.odd to say,im drawing 11 amps on start and run wires coming from the contactor but i hear nothing from the compressor! any suggestions? Lou

Reply:

Check first for a bad start relay.

Outdoor unit won't run: Is the problem with the fan motor or the compressor motor in my condenser unit?

I've a Lennox outside a/c unit. The fan/motor and compressor are part of the unit. The fan was not spinning and the A/c is not running. I've isolated the problem down to the fan/motor or the compressor, but don't know how to check or find out which it is.

Any idea on how to check if the fan/motor is 'bad' or the compressor is 'bad'? - Fan or Compressor?.

Reply:

Fan or compressor: you could also have a bad control board, or relay switch in the compressor/condenser unit

If the fan is bad and the compressor is OK you'll hear the compressor turn on at the start of a cooling cycle, though a bad fan and system overheating could later cause some compressors to turn themselves back off.

Similarly, if the compressor is bad your fan motor would still run. So I'm suspecting a control or switch problem.

Old RUUD air conditioner condenser unit won't run: diagnostic tricks

I have a very old (>27 years) RUUD a/c with an emerson 1/3 HP 4-wire PSC motor. A few days ago I noticed the fan was not running outside. Fan would not spin easily with a stick with power on or off. Decided fan motor must be bad.

Took it out, still wired. Upside down, spun it easily by hand. Put circuit breaker back on; fan motor humming loudly; so decided motor OK, but start capacitor was bad. Got one next day and reassembled. No improvement.

Pulled fan motor out of cage again. Still wired, upside down on top of unit.

Now with power on, I COULD spin it up and it seemed to operate OK. Tried putting it back right way. Now, nothing. Checked circuit breaker again.

Two pole 60 amp Square D; one side open in either position (I remember replacing circuit breaker about 25 years ago).

Had to order, so another day gone. Got it yesterday. Now fan started itself when AC powered (but still upside down on top of unit), so I figured it was fixed with new capacitor.

Reassembled, fan now in correct position. Back to not starting itself, plus powered fan difficult to turn again, humming loudly. Started checking all wiring, turning fan over, etc. Managed to pull couple wires loose.

\(Previously, they didn't seem to match all the on-line help about how to wire.) Think I got all back where they had been when it was working in past, but not positive.

Long story shortened -- fan works way it should when it is upside down; sounds like it is fighting with itself when right side up.

What gives? Also, noticed a little up-down play in shaft, but none side to side. - On 2020-07-19 by elrod

Reply by (mod) - diagnostic steps lead to bad fan motor or bearings

Elrod

That's an interesting and helpful series of diagnostic steps you tried. I suspect that the fan bearings are shot, so when the motor is in proper position the shaft is binding.

Followup by Elrod

Thanks, I had two other opinions this morning that offered the same thoughts, because of the vertical play. Except one thought it was probably electrical (position relative to brushes) and not mechanical (binding) because the effect was worse when under power.

I might keep the motor and eventually do a post-mortem autopsy on it to find out for sure (but this is definitely low priority for the near future).

Either way, the consensus seems to be the motor has reached its EOL, being more expensive to rebuild than to replace.

Now, if I can just be successful in removing the STUCK fan blade from the shaft (even a gear puller didn't help, was just bending the fan flange our of flat -- I see more work ahead of me for that).

My appreciation for your taking the time. Maybe several of us learned from this. Stay safe.

Reply by (mod) -

Elrod,

That post mortem would be both interesting and helpful to others, so if you do it, please tell me what you find.