Air conditioning thermostat (C) Daniel Friedman Air Conditioner / Heat Pump Won't Start: Diagnostic FAQs
Questions & answers for A/C or Heat Pump that won't start

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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.

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

Damaged air conditioner electrical wire (C) D Friedman T Hemm

Question: My Air Conditioner won't turn off - what to do?

Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?


Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system.

If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician.


Question: My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems

Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!

Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old

Five causes of a home air conditioner compressor short cycling on and off too rapidly - Short Cycling Air Conditioner Diagnosis & Repair

DanJoeFriedman (mod) said to Joe and Matthew (re questions just above):

  1. Loss of air conditioner refrigerant - a refrigerant leak in the system. You may be able to repair the short cycling problem temporarily by recharging the air conditioning system but the proper repair is to find and fix the leak. Otherwise you will have to keep repeating the costly service call to just add refrigerant.
  2. A/C Coil Icing - the evaporator coil (cooling coil) is iced over (such as due to improper refrigerant charge or dirt or a reduced air flow due to a dirty filter) - take a look in the air handler to see if the coil is blocked by ice or dirt.
  3. Oversized air conditioner - if the short cycling has always been a problem since the day the system was installed, there is a good chance that the unit is too big (too many BTUS) for the space being cooled. A more subtle version of this same problem is that you've done something like closing doors or adding a partition that had the effect of reducing the size of the space being cooled. Sometimes we can mitigate this problem by running the blower fan at a lower speed or by opening interior doors to increase the size of the space being cooled or even by moving the thermostat.
  4. A/C control problem - it's less likely, but a damaged control board or switch could also be causing rapid equipment on-[off cycling.
  5. Compressor damage or compressor start-troubles: - I've seen these other causes of air conditioner short cycling: if someone manually turns the air conditioner thermostat up and down or on and off too frequently, a hard-starting compressor may find that it has been shut down with high internal head pressure inside the compressor. Normally that head pressure bleeds off over time, making it easier for the compressor to re-start (against low head pressure) the next time it turns on.

But if someone is turning the system on and off quickly, the compressor may have a hard time re-starting against the pressure on its outlet side. A starter capacitor addition or replacement might fix the problem. If your A/C compressor is showing this symptom but works OK if you leave it shut off for 30 minutes or longer, that may be the trouble.

We have also see or a damaged compressor internal refrigerant valve causing high head pressures;

In sum, you need a service call from a professional to correctly diagnose and repair the problem. Ask the service tech what she/he found and let us know - what you find will help other readers.

If your air conditioner or heat pump has the opposite problem, staying on too long,

Question: Why is my A/C not producing enough cool air?

A/C not producing enough cool air - I had my air conditioning system gassed up last week - $210. ! The unit is not producing enough cold air. The unit is set on 72 and does great at nights but during the day where the temp. outside is reaching mid 90"s it's getting up to 80 in the house. Is my duct work screwed up? How do you repair trailer metal ducting? - Amanda

90 degrees outside and 83 inside with thermometer at 76. Cools off to 76 when sun starts to go down and house then gets cold. New capacitor and condensor just put in. Help. - Ginny 5/17/12


Amanda: if your system is not cooling there could be any of a number of problems - see the article above as a place to start. If your basic complaint is that the A/C temperature at the supply registers is cool enough but the volume of air flow is too weak, we'd start by:

Weak air conditioner air flow - is there a relationship between refrigerant charge level and weak cool air flow rate?

Les said: Weak air conditioner air flow: Our A/C was serviced two months ago and the repairman said it had a leak. $400 later it was recharged with coolant and now the ac is doing the same thing. Very little pressure coming out of vents and no cold air coming out. Does anyone know what I can do for the weekend? It is stifling!!!! See A/C Air Duct Problems

DanJoeFriedman (mod) said:

Les: A leak that was fixed by a re-charge is not as good a repair as a leak that was fixed by finding and fixing the leak - you'll just have to keep adding refrigerant.

But weak air flow out of the vents would not be due to a refrigerant leak; more likely a clogged filter or crushed or disconnected ductwork, or a blower fan problem.

Weak air Flow traced to Clogged Air Filters: Can Clogged Air Filters Affect the Room Thermostat?

(May 13, 2011) Jim said: If air conditioner filters are clogged will it cause the thermostat to shut off?

Jim: clogged A/C filters won't cause a room thermostat to shut off. The thermostat responds to room temperature. However clogged A/C filters that reduce air flow, cause coil frosting, or otherwise reduce or stop the flow of cool air into the room where the thermostat is located would mean that the thermostat would remain "un-satisfied" and should mean that the thermostat says "on" - continuing to call for cooling.

Air Filters & Cooling System Amperage Draw: Could Removing the Air Filters Cause the Electric Motor in the Air Handler to Draw Higher Amps?

Question: got one for you. i put an ammeter on my air handler and it read 8.25 amps, I removed the filters and it went up to 9.75 lmao at the situation the amperage should have gone down. what gives here

Reply: causes of variations in electric motor efficiency and current draw measured in amps

Lost: this amps variation is beyond my expertise, but in general reducing the load on an electric motor will show up as lower amps or current draw, not higher amps. Here are two interesting explanations of amps or current variations on an electric motor that I found when researching the question:

1. Voltage variations and current draw at electric motors: If your supply voltage is varying from your power company that can show up as higher amps draw on the motor (though it's a suspicious coincidence to see it exactly when you removed the filters and supposedly reduced the load on the motor). Quoting from motorsanddrives [dot] com: "The effect of low voltage on electric motors is pretty widely known and ... The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same ... To summarize the situation, low voltage can cause high currents"

2. Load variations and electric motor efficiency: A second possible source of seeing higher amps or current draw on your blower motor when you pulled out the air filters and thus reduced the load on the blower motor might be illuminated by this U.S. DOE pamphlet "Determining Electric Motor Load and Efficiency" - Quoting: "Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50% load."

Question: what causes warm suction lines in an air conditioner or heat pump system? Warm A/C suction line (should be cold) and zone control dampers that stopped working

JMONTE said: warm A/C suction line question: After my condenser is turned on for about 4 minutes the suction line starts to get warm to the touch. can you tell me what the problem may be


JMONTE: If the HVAC suction line gets warm, you may be out of refrigerant, or the system may be running in heating mode if it's a heat pump.

Becky, If a motorized HVAC zone control damper is not opening or closing, most likely the motor has failed, or the thermostat that operates that zone control is off or set in an incorrect position. See ZONE DAMPER CONTROLS

Becky said: motorized air conditioning zone dampers not working

My house has "zone" control with dampeners to close off the upstairs over the garage room unless that thermostat is on - the room is not cooling. I have located the damper under the house. What are some causes for the damper not opening and how to repair them? condensation, motor to damper? silicon glued properly?

Question: first check circuit breaker if AC or heat pump won't start

(Aug 23, 2014) Anonymous said:
My goodman CLJ30-1 ac wont turn on I have power to everything but nothing wants to come on curious if I should be getting any voltage readings of 220+- anywhere and if so where because I don't and what could be the possible problems keeping it from turning on

(Aug 24, 2014) Dan Haney said:
My House has central air, this morning I try to turn down thermosat for the A/C to kick on, but nonething happened, I even try to turn the air conditioning cntrol switch to fan nonething. could it be that my a/c fuses are blown?



I'd start by checking the fuse or circuit breaker, then check for power at the unit

Question: A/C or heat pump blower fan won't start

(Dec 13, 2014) Anonymous said:
I have a oil hot air furnace,the gun starts but blower won't come on,replaced blower control panel,need some advice,


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.

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

(June 2, 2015) Bobby said:
The indoor unit has power to the furnace transformer and the transformer is producing around 27 vac but there is no power at the low voltage terminal block on the furnace circuit board. and neither the outdoor or indoor unit starts when called.


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

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

(June 4, 2015) Anonymous said:
Fan on my A/C unit won't start by itself, but if I turn on the unit and spin the fan by hand to start it, itwill run on it's own. What could be the problem?


Anon: look for and replace the fan motor start/run capacitor.

Question: A/C not cooling after basement flooding

(June 30, 2015) Evelyn said:
Our basement took on water. The a/c quit working. Husband changed the fuse. Fan runs but no cool air.


If the blower unit runs but there's no cool air most likely the problem is with

lost refrigerant

your outside compressor/condenser unit

or with a thermostat or control not set correctly indoors.

Ask your HVAC technician to include, in her diagnostics, a check for a control that got wet and was damaged.

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

(July 12, 2015) A/C Concern said:
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?


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

(July 12, 2015) A/C Concern said:
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.

Question: heat pump stopped blowing cool air

(July 26, 2015) Bob said:
Twice this summer, the upstairs heat pump stopped blowing cool air (the fan continued to circulate air, and the thermostat remained on but the air would not reach set temperature), but the unit started working again fine the next day. Both times we were dealing with a hot day (90-100, although other days this summer were also that hot). Is this off then on behavior symptomatic enough to diagnose the problem? Thanks.



We need an onsite inspection and some tests to figure out what's happening with your system. It may be something as simple as a dirty air filter that causes the cooling coil to ice over when the system is working hardest.

Question: Rheem RANL-037JAZ won't start

(Aug 4, 2015) P.H. said:
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.

I found the bottom of the run capacitor (the side with 3 terminals) is rusted, the whole thing is under the metal case cover, and the whole outside unit was under protecting cover too, I have no idea how that capacitor can get rusted, do you think that run capacitor is bad and even the booster can help, or is there any other reason? Thanks.


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.

Question: heat pump won't start or run in A/C mode

(Aug 6, 2015) Gary said:
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 15, 2015) Ella said:
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...



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.

What unit, Ella? Is this a central air or a wall/window unit?

Search for DIAGNOSTIC GUIDES A/C / HEAT PUMP or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

(Aug 17, 2015) Ella said:
It is central air. After talking to people and checking several things I think it may need coolant but don't think I can do that myself. it was fine last tuesday-then didn't come on sat. Would be grateful for any pro bono help... as it is/going to be in the 90's this week i am hoping to stay out of the emergency room for heat stroke like i did 4 years ago-once i got back in a.c. i was fine. Been all over the internet reading everything.


If an A/C unit does not turn on, that is not itself a refrigerant problem. It's a problem with the thermostat, wiring, controls.

AIR CONDITIONER WON'T START is where you should start

(Aug 18, 2015) Ella said:
There are no switches visible outside on the compressor-Aren't those wires to the control board and/or the air handler inside the wall? Neither is visible or if visible, identifiable as such. All simple stuff has been done-fuses, anything visible-settings-have tried setting it 6 degrees below current inside temp.,nothing happens-the thermostat is working enough to tell the temp.,what setting its on,& other useful info-Even if I could get the cover off the compressor/condenser,(a hot sweaty job at best with only screwdrivers even in the earlist of mornings)I wouldn't know what to look for. I have hooked up numerous vcrs,dvd players,routers,tivo boxes and put together Major pieces of furniture from IKEA By My Own Self so i am not a total dunce at mechanical things but this is throwing me for a loop. May well be the half-assed way the #@!$%^&*+! contractor installed things-he certainly avoided letting me know where anything was downstairs...

Ella, AIR CONDITIONER WON'T START is where you should start reading.

(Aug 18, 2015) Anonymous said:
Ive been there. It gives me nothing I can use. Please answer the questions/issues I have raised after reading that...


Ella I really want to help but can't make much out of your question.
You are describing a problem of an A/C that does not turn on. That's all you've said.

The article I cite gives a step by step of what to check when an A/C unit does not turn on when you want it to.

If you did each of those steps and still are left without a clue, it's time to call an HVAC professional service technician.

An HVAC service tech would check that there was power being delivered to the equipment, and that the thermostat was calling for cooling. A check at the thermostat terminals at the air handler and compressor condenser follows, then a check of relays, control boards.

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

16 Aug 2015 kinsey said:
My Haier thermocoool only shows the green lights/readers and then turns off tuns off immediately. What can be the cause?

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.

Air Conditioner or Heat Pump Won't Start or Won't Run Articles


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