Erin Doman on May 30, 2016 38 Comments HVAC Troubleshooting: How to Fix An HVAC Compressor That Won’t Turn On If your air conditioner is not working, the problem can often be pinpointed to the HVAC compressor. This is not necessarily a death sentence for your HVAC unit. Many small, inexpensive and easily replaceable parts are prone to breaking down. Other times, your HVAC may simply need a thorough cleaning. The problem is often simple enough to be repaired by an average homeowner. Walking through these simple steps will help you determine what to do when your HVAC compressor won’t turn on, and decide whether or not you need to bring in the assistance of a certified HVAC technician. What is an HVAC Compressor? Your compressor is a critical part of your HVAC system. In residential units, it is typically located as part of the outdoor unit. The role of the compressor, true to its name, is to compress the refrigerant fluid as it carries heat out of your house. This heats up your already warmed refrigerant fluid even further, converting it from its gaseous form into a liquid. Though purposefully adding further heat may seem counterintuitive, it is important that the refrigerant be at a higher temperature than the outdoor environment. Especially if you live in areas where summer days can reach up to 120 degrees Fahrenheit. This taps in to a basic rule of physics: Heat will always transfer from warm to cool areas. To ensure that the heat your refrigerant fluid is carrying outside is transferred efficiently out of your home, it is crucial that your compressor be well maintained and in working order. Maintaining the Smaller Pieces The good news is that it may not be necessary to replace the entire HVAC compressor. Your compressor is made up of multiple parts which can be replaced individually. If it is one of these parts that is failing, simply replacing the smaller component can grant you significant savings. Check Electrical Wiring If your compressor will not start, your first step is to check and see if your unit is receiving power. Examine all the wiring in your HVAC unit, especially the wiring which connects it to an outlet. If you have a blown fuse or a tripped breaker, this may be the end of your search. It is time to bring in an electrician who will help you decide what to do when your HVAC compressor won’t turn on. Easy Fixes First Begin with the quickest and cheapest maintenance and cleaning tasks, just to rule them out. Before you do anything, make sure you fully shut down your unit. You can do this by pulling the disconnect block out of the electrical box next to the condensing unit. If you have a voltage sniffer, check inside the box to make sure that the power is off. Buildup of dirt and debris in the evaporators, filters or condensers can cause your entire unit to shut down. A clogged filter or register can reduce the airflow so much that your evaporator coil will ice up and stop functioning altogether. Regardless of whether or not this is the source of your current HVAC compressor shut down, keeping this area clean should be a part of your regular maintenance routine. A clogged condenser coil can overheat the compressor. In the best case scenario, the compressor will simply shut down before it is damaged. In the worst case scenario, continuous overheating can damage the compressor so much that it may need to be replaced. Go ahead and clean your condenser coils, replace or wash your filters and make sure the supply vents are clear of blockages. If your compressor still does not turn on, it may be time to bring in an HVAC technician. A technician will test out the following parts to determine what to do when your HVAC compressor won’t turn on: Capacitor Start relay Valves Terminal connections If you have a certain amount of electrical know how, you may want to consider replacing your own capacitors and starter relays before you bring in outside help. If a capacitor or starter relay replacement still does not fix your problem, it is definitely time to bring in a professional HVAC technician. More complex repairs may involve dangerous chemical leaks and other safety hazards which are best handled by trained professionals. Your Capacitor and Starter Relays Ask any technician, and they will tell you that about 85 percent of HVAC repairs involve electrical systems. Your capacitor and starter relays are critical in providing power to your compressor. These are the items which should be examined first, as they are the most likely to fail. They are also the cheapest and the easiest pieces to replace. If you have basic electrical knowledge, you may consider replacing these parts yourself. If your unit is more than five years old, it may be time for a whole replacement to head off any future problems. Your HVAC capacitor is a short term power supply which starts and runs the compressor, as well as the blower motor and the outside fan. Your starter relay transfers power from your capacitors to your compressor. If your HVAC will not turn on, but you hear a humming sound from your outdoor unit, it is likely that this is the compressor attempting to access a non-functioning capacitor. Turn your unit off immediately. This can overheat your compressor, causing unit wide damage. If left unattended, this can elevate the situation from a simple capacitor replacement to a need to replace the entire compressor motor. Unlike a capacitor or starter relay replacement, a full compressor replacement should only be performed by a trained HVAC technician, which makes this repair substantially more expensive as well as more time consuming. A Dead HVAC Compressor If the replacements do not bring your HVAC compressor back to life, you have two options. You may have a technician replace your compressor, or you may decide to replace the entire air conditioning system altogether. If the cost of a replacement is less than half of the cost of a new unit, this is most likely your best option. Once the replacement price exceeds 50 percent, it may be time to purchase a newer unit. Troubleshooting Your Malfunctioning HVAC Compressor You don’t want to be stuck in the heat of the summer with a malfunctioning HVAC compressor. As soon as you notice a problem with your HVAC unit, you should call in a professional HVAC technician to take a look and make any repairs and updates that they deem necessary. Replacing smaller parts as they need them will prevent you from having to replace your entire HVAC unit prematurely.
Kylie Dotts says June 26, 2017 at 5:32 pm Interesting how you said that if the replacements don’t bring the HVAC back to life you may have to replace the compressor or replace the whole air conditioning system. We’ve been experiencing some problems with our air conditioning unit over the past few days and are planning on calling a repairman to come and take a look. I think we’re going to have to find a compressor replacement so maybe we’ll just pay him to install it while he’s out here. Reply
Gordon Watkins says July 2, 2017 at 3:52 am I know my run capacitor is bad and can replace it myself. But I need to know why it failed in a 1yr old unit. And if I replace it, will it just go out again or will it damage something else like the compressor. Reply
Chirpa says July 8, 2017 at 1:14 pm Sometimes capacitors just go bad…maybe it was a faulty one to begin with…sometimes it may be that your unit gets very hot outside and it causes damage to the capacitor…in any case, replacing it should be inexpensive and hopefully this one will last a longer time… Reply
Jesus says July 18, 2017 at 12:36 am Hi,I’m having a problem withy a/c ,it has 1 dual capacitor that runs the fan motor and compressor motor on the outside unit,the fan motor starts but Not the compressor,my question is Can the capacitor be partially bad and just run one But not the other motor? Thanks I’ll hope someone gives me a answer it will be an easy fix if that is the case Reply
denis says October 9, 2017 at 11:38 am Yes, you just described a very common problem. Simply replace the capacitor ($10-$20) and your unit will be fine. Reply
Josh says August 30, 2018 at 9:31 pm Same thing here, I pullled off the wiring schematic and it only has one capacitor in the line, my condenser fan will start and run anywhere from 5 hours to two minutes, I think the capacitor is still good because it still cranks up the blower motor and the compressor, I’m gonna drop in a new blower motor tomorrow because it feels hot to touch and is making tha fried electric motor “hum” and see what happens, if that doesn’t work I’m shelling out for a whole new Ruud system, its august in south Florida, feels like the hurricane all over again Reply
ross salupo says July 19, 2020 at 6:57 am yes one side can fail your best bet is to try a new dual voltage capacitor if you know what you are doing. Turn power off to unit at main disconnect and at thermostat because you could still have 24 volts going out to unit.And remember if you touch top of capacitor without discharging it you can get shocked from it because it is like a battery.Replace capacitor and see if that resolves your problem with the same rating dual capacitor that you removed it will be rated in microfarads and will have a uf after the ratings like for example 35uf 5 uf etc 370 vac or 440vac. If that doesnt solve your problem your last ditch effort if compressor is mechanically stuck not electrically failed would be to try a hardstart kit. Reply
Vernon Smith says July 27, 2017 at 8:47 pm Could have been a power surge , lightening..etc. that caused it to go bad. A faulty relay might have caused it to overheat too. Its possible they put a wrong capacitor at the factory. Make sure you know the vac value. If it calls for a 400 you need one of that or greater value. But if it calls for a 400 you can’t use a 370. If it calls for a 370 you can use a 400 tho. Reply
Blake says July 29, 2017 at 5:59 pm I have a mini split unit with a 50 ft line set. Will no or low freon cause the compressor not to turn on? This is a brand new unit. Reply
ross salupo says July 19, 2020 at 7:01 am Yes it will more than likely have a low pressure and high pressure cut off switch to prevent damage to the compressor. Reply
Floyd HVAC says August 3, 2017 at 4:27 pm A lot of the cheap capacitors are just that…cheap. get an american made like Triton if you can find one. They last. Reply
Neil says August 16, 2017 at 7:13 pm A bit late, i know, but next time call a 24 hour HVAC repair company. You can buy many parts off of the technicians and almost all have the most regularly used capacitors on hand in their trucks. Most are happy to meet you and sell you won. You won’t like the price compared to it’s value, but it’s not terrible considering you wont have to wait a day or two for it. I’ve heard $50 – $65 some other people had to pay, but my tech charged me $22 to come buy one off of him. Double the retail price, but at night on the weekend, I wasn’t complaining 🙂 Reply
denis says October 9, 2017 at 11:40 am Most A/C techs will charge over $100, sometimes $200 if it is after hours. for a 10 – 20 dollar capacitor, just wait and buy one from an electrical store with the exact same specs. Reply
Danny says October 13, 2017 at 3:26 pm My compressor was not turning over at all so I replaced my capacitor. Now, my compressor turns on, but then quickly shuts off. Any idea on what may be going on? Thanks Reply
Jamie says November 9, 2017 at 7:14 am For a service van to come to your house it has already cost the company 100 dollars…it may be a 10 or 20 dollar cap but the knowledge and overhead isn’t free….if you don’t like paying that kinda money out then crack open the books and learn how to do simple troubleshooting and repairs yourself Reply
Jamie says November 9, 2017 at 7:16 am You would need a set of gauges but it sounds like it could be low pressure cutoff or the compressor could be toast and is drawing way to much and overloading…..in this type of situation it’s best to call a pro….feel your compressor is it really hot? Reply
SB20 says November 29, 2017 at 2:11 pm Is it possible for the compressor fan to run but not the compressor itself? Reply
Steve says September 28, 2019 at 4:14 pm Yes it is. That’s what I was experiencing. I could get the fan to run until the fuse would blow again. But the compressor wouldn’t come on because it was failing. This was verified by removing the cap from the compressor with all power disconnected at the fuse, breaker and wall. Then testing the continuity between one of the terminals to ground was positive/beeping which in this case was bad. 🙁 Reply
Tommie says February 26, 2018 at 3:24 pm I have a heat pump and the compressor comes on in a/c mode but does not come on in heat mode which makes me think im using only heat strips for heat .. very exspensive that way… any thoughts Reply
marvin ls sr says May 3, 2018 at 4:50 pm i have a 4-ton Goodman unit and the contactor failed and the unit quit. replaced it and nothing. after replacing the capacitor, again nothing. rechecked the contactor and there is power only incoming and nothing going to the capacitor so, the contactor will not engage…help.. Reply
Peter M says May 10, 2018 at 6:52 pm My Lennox works only when I physically engage the contactor strip (i.e. the magnetic coil does not pull and hold it closed). Contactor is new and I have ordered a new capacitor. Hope this is the cure. Comments? Reply
Vincent Armstrong says May 24, 2018 at 2:52 pm My compressor is trying to turn on but shuts back down immediately after trying and the fan still turns on any advise would be much appreciated Reply
John says May 28, 2018 at 12:56 pm Hi, My central A/C was working perfectly this year. When I woke up this morning, the house was hot and humid. The A/C was no longer working. When I checked the furnace breaker, it was OK. The furnace blower will not start, even in manual fan mode. Could the blower itself be bad and causing the A/C compressor NOT to kick in? And, is there a relay or something to test for the blower motor? Any help would be greatly appreciated as it’s supposed to hit near 100F today. Thanks! John Reply
Sly says June 9, 2018 at 4:33 pm My ac evaporator comes on but as soon the compressor is to be energised, the whole system will shut down and start again the same cycle Reply
Nelson.lopez says June 11, 2018 at 1:24 am Hello, I replaced my Run start capacitor. I see my fan is working the contact engadge I hear the compressure turning on then stop. Any ideas? Reply
J.b says June 18, 2018 at 1:45 am Question my ac unit will shut down but my fernece fan keeps running what’s causing the problem Reply
Villie Frank says June 18, 2018 at 1:38 pm Outside trane a.c. unit will turn on then off for a few seconds then back on.It does not do this on every cycle.Just now and then.Air continues to blow inside and cooling well. Reply
Scott E Demore says July 14, 2018 at 11:01 pm for the last couple of days the temp in the house has been going up slowly and ac running constantly. Today my compressor quit. I checked the heat pump outside. Its getting power but the cooling fan outside will not turn on. When I turn ac on, I can hear a humming comeing from heat pump but no fan. Any thoughts? Reply
Rehman, K S says July 31, 2018 at 12:36 am I have the same problem as Scott Demore,but my compressor motor make a whining sound after some time and my unit is just 2 years old. I talked to my technician who installed the unit, he said it may be simple capacitor gone bad. He promised to come tomorrow and have a look. I will see the results the unit is still under warranty. Reply
Eric says August 1, 2018 at 8:34 pm I have a new Bryant that isnt even a year old. I’d say 3-4 months old and the compressor Hums a split second then clocks off yet the fan still runs. Not a very common problem regarding brand new units. contactor and capacitor are working like they’re supposed to. Any ideas? Reply
RAY . says August 6, 2018 at 10:49 am my 5 ton unit work fine for a while at the end of the day when it start again the fan outside and inside the house goes on but the compressor do not go on i will turn everything off for 15 20 minutes then it will start again why i had thecnician coming but he do not seem to know the probleme my unit is 8 years old Reply
Michelle Washnigtonr says August 9, 2018 at 7:11 am My AC stopped working. The fan on the outside unit was not running. The capacitor was replaced and the air started to flow but I was then told the compressor was shot. Is that possible? Reply
David Kech says September 19, 2018 at 10:28 am My Lennox AC unit quit cooling.Replaced the compressor,capacitor,TXv.Still nothing.Now they say this compressor is no good also.Any ideas why the compressor would be no good.Also how do you blow out the lines from the compressor Reply
Patrick A Yacks says September 15, 2020 at 2:06 pm If the starter motor starts but the compressor but the compressor doesn’t. Will that draw an excessive amount of amperage. Reply
