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Problem: My outdoor air conditioning unit’s high pressure switch trips ever so often. Why does the high pressure switch trip so often?

Problem: My outdoor air conditioning unit’s high-pressure switch trips ever so often. Why does the high-pressure switch trip so often?

Answer: The high-pressure switch tripping on an air conditioner or heat pump unit can be caused by several things:
1. Dirty outdoor coil. Turn off the power and clean with a garden hose.
2. Slow or dragging fan motor. Replace motor or capacitor.  I would recommend testing the capacitor first. I would need to know the capacitor specs (uf/mfd and voltage rating) to recommend a replacement.
3. Motor or fan blade going the wrong direction. Air should come out of the top of your unit.
4. The system is over-charged with refrigerant. Would need gauges to check this.
5. Faulty high-pressure switch tripping without having high pressure. High-pressure switches are usually set to trip at a little over 300 psi. Sometimes I have seen switches trip for no reason at all.

6. A blockage in the refrigeration cycle. This could be a TXV valve or metering device that is partially stopped up or not working correctly.

7. Extreme high outdoor temperatures above 100 degrees. Your outdoor unit has to be clean to run properly under high outdoor temperatures. I would recommend that your unit is thoroughly cleaned and charged properly. If you have any other questions please feel free to email us at support@arnoldservice.com or comment in the comments section below. We would love to answer your questions and have your business!

We hope you can easily find and fix the problem. We would love to help you out and earn your business! Steve & Barbara Arnold

32 thoughts on “Problem: My outdoor air conditioning unit’s high pressure switch trips ever so often. Why does the high pressure switch trip so often?

  1. The control board on my ac/heat pump shows a high pressure switch fault. I had a problem with the unit blowing the 3 amp fuse when switching to heat. I replaced the fuse and cleaned all the coils and it appeared to be running fine in ac mode and heat mode. But that is when I discovered the 02 fault which the book says it is the high presure switch. Do I need to replace the switch even though the unit appears to be running ok. The unit is 3 years old and is a nordyne unit.

    1. Hi David! I am sorry, I am not familiar with high-pressure switch operation on the newer heat pumps. Hopefully, since you cleaned your unit the pressure switch fault will not occur again. If the high-pressure switch continues to flash the high-pressure switch fault code then I would like to suggest that you or a service tech install gauges to see if the pressure is actually high or if the pressure switch is bad. On the older pressure switches, they were brazed into the refrigeration lines and they are impossible to replace without reclaiming the refrigerate charge, brazing in a new high-pressure switch and liquid line filter drier, evacuating the system to get any air or moisture out then recharging the system. This can be very costly and labor-intensive. I would recommend that you have your heat pump checked out to make sure that you really are not running high pressure so it does not ruin your compressor. If it is a high-pressure switch that is bad then I would suggest replacing it only if you do not have to break open the refrigeration system. Anytime air, moisture or dirt can be introduced into the system then this is not good. If the pressure switch is an easy replacement then I would do the replacement only if the high-pressure switch is actually bad. Most of the time high-pressure switches do not go bad very often. I hope you can find out if you really have a pressure switch problem or not soon. You make want to turn the power off to your unit for 30 seconds or so and see if the flashing code resets and goes away. I hope that it will go away. Steve

  2. Hi Steve
    I looed at a heat pump today and found it shutting down on the high pressure switch. It is a new installation and appears there is not enough cfm passing through this 3 ton unit ( by my calculations only about 900cfm) – Would or could that cause the high pressure situation? Never seen this before – it is in the heating mode. thank you in advance for any help you can give – Dennis

    1. Hi Dennis! If you are in cooling mode with the new outdoor unit is going off on the high-pressure safety switch then I would think that your unit is over-charged or you have a restriction in the refrigeration cycle. About the only way that I can suggest to tell what is going on with the unit would be to install gauges and see what the pressure readings are. Is the pressure actually high or is there a problem with high-pressure safety switch or defrost control board. Hooking up gauges would tell you if the TXV (thermostatic expansion valve) is causing the problem. 900 cfm would not be enough if you are talking about the indoor unit having only 900 with the unit in the heating mode because as you know it should be 440 cfm per ton and 900 would not be enough for a 3-ton unit. I hope that you can hook up some gauges, find and fix the problem. Steve

  3. an a/c guy disconnected the high pressure switch on our outdoors 2002 Ruud unit because it kept tripping and he said the switch was not needed. yesterday we had a bad storm go through and the electricity went out several times. then after storm passed unit would not cool. I went outside and the top of the fan blade area was extremely hot and blades were not moving. what happened? did not having the pressure switch have anything to do with it? he came out again last night and says that the compressor etc needs replacing and it is going to cost another $900.00 after last years $795.00 he charged to work on it and when he disconnected the high pressure switch

    1. Hi Danna! So sorry to hear that this happened to you! If your unit was kicking off the high-pressure switch this is a good sign that either the high-pressure switch is bad (which is unusual) or your unit is dirty, over-charged, has an obstruction if the refrigeration system or has a slow fan motor. Since the fan motor went out after the storm this is a sign that more than likely the fan motor was going bad, moving slow and was causing the high-pressure switch to go off. If the service tech installed gauges to test your system then he should have seen the high-pressure problem. The service tech was probably trying to prevent you from having trouble with your AC not working, by disconnecting the high-pressure switch. This is not advised because this protects from hurting the unit. The unit should not have been running in a storm. I always recommend turning AC units off during storms because this can be fatal with power going off and on, power surges and lots of water. I will let you decide who is at fault. I do not like to take sides in these kinds of situations. So sorry that you are out of AC! Steve

  4. My outside heatpump at time will drop out the compressor, even though the compressor contactor is pulled in and a volt measuring indicates 240 on compressor side of contactor
    The compressor was recently replaced as were the caps. When running the compressor is drawing a around we amps the start wire has 6amp on it.
    So I’m wondering why the compressor stops running. When it is running make very good heat or cooling.
    It some times runs several days without any symptoms.
    I’m starting to wonder if the reversing valve is getting erratic. What do you think?

    1. Hi Loren! My best guess would be that the compressor is over-heating and going off on thermal overload. I would like to suggest that you turn off the power to the unit and touch the top of the compressor to see if it is hot the next time the compressor cuts off unexpectantly. If the compressor is hot then this could be caused by a dirty condenser coil, a fan motor that is cutting out or running slow, (not up to speed) running low on refrigerant, a restriction in the refrigeration cycle or a faulty TXV valve. Many of these problems can be determined by installing gauges and looking at the pressures to see what is wrong. I would also recommend turning off the power to your unit and check all wiring to make sure everything is good and tight. I would highly recommend taking the compressor terminal cover off and inspect the 3 wires that go into the compressor. Make sure they are tight and there is no burning in those wires. I hope you can easily find and fix the problem. Other than those things I have no idea. Steve

  5. Hi, I recently purchased a new house with a carrier ac unit. this is our first summer and the ac is not working. The psi level is at 700 which is extremely high for a 3.5 ton unit. Do you have any recommendations on what the problem could be? the unit runs for 30 mins then it stops blowing cold air.

    1. Yes, this is extremely high and should not happen. This sounds like you have a stoppage in the refrigeration system somewhere. There have been lots of problems with TXV valves so this could be the problem or you might have a fan motor that is running too slow. If the fan motor is running slow then I would recommend checking to make sure that the fan motor capacitor is in good condition and up to specs. I would recommend that you have a tech install gauges and find and fix your problem. 700 psi is very dangerous and could cause leaks in your system. Steve

  6. hello steve
    I got a System with 134a that will run for about 2 seconds and then trip on high pressure. I gauged it up and it trips at 400 psi. Any ideas on issue?

    1. Hi Carlos! This sure sounds like you have a restriction in the system. I know that several manufacturers have been having problems with TXV valves getting stopped up, but I do not know for sure. Here is a link to a good video on how to determine a bad TXV: https://www.youtube.com/watch?reload=9&v=L7VkLsXGaQ8 The AC Service Tech Youtube Channel has many videos that should be able to help you out. I am sorry, but sometimes it is hard to find out where the restriction is located. I hope you can easily find and fix the problem. Steve

      1. Thank you steve,
        My other thought was the compressor since this unit has been running for a years with no issues till no. I just found it weird that it will trip so quick on high pressure.

      2. Hi Carlos! Yes, that is weird that it trips so quickly on high pressure. I had a Copeland scroll compressor that did this and it turned out to be a stuck check valve that was in the high-pressure discharge line of the compressor. It was located right outside the compressor’s discharge line. I could not believe that happened and I had to replace that valve to fix the compressor. I hope that is not your problem and it is something that is an easy fix. So sorry for your trouble. I know it can be a pain trying to figure out what is wrong. God bless you. Steve

  7. Hi Steve,
    I am working in an industry on a HVAC system. The outdoor temperature is around 46 degree Celsius right now.
    And we need the HVAC to be working at any cost but the problem is that the compressor often trips with high pressure alarm. Is there any specific reason to that?

    1. Hi Abhishek! Here are the reasons that I list in this post. The following are the only reasons that I can think of that would cause a high-pressure alarm to sound:
      1. A dirty outdoor coil. Turn off the power and clean with a garden hose. 2. Slow or dragging fan motor. Replace motor or capacitor.  I would recommend testing the capacitor first. I would need to know the capacitor specs (uf/mfd and voltage rating) to recommend a replacement. 3. Motor or fan blade going the wrong direction. Air should come out of the top of your unit. 4. The system is over-charged with refrigerant. Would need gauges to check this. 5. Faulty high-pressure switch tripping without having high pressure. High-pressure switches are usually set to trip at a little over 300 psi. Sometimes I have seen switches trip for no reason at all. 6. A blockage in the refrigeration cycle. This could be a TXV valve or metering device that is partially stopped up or not working correctly.7. Extreme high outdoor temperatures above 100 degrees. Your outdoor unit has to be clean to run properly under high outdoor temperatures. I would recommend that your unit is thoroughly cleaned and charged properly. I hope you can easily find the problem and get it fixed soon. Steve

  8. Hi Mr. Arnold – I have an old Tempstar 5500, Model CH5524VKA2 installed 1993. It’s our 25th Anniversary! The low pressure sensor on it is stuck open so the heat pump won’t start. It looks like the sensor can be easily replaced by unscrewing a nut but I’m worried that the refrigerant will escape. Would you know if there’s a schrader valve in there so I can do it myself? While I’m there, I’m thinking to replace the coil’s temperature sensor which looks pretty easy. But there’s this dense, black material around it and I’m wondering if that’s for insulation or just to keep the sensor in place? Thank you!

    1. Hi Gary! So sorry for the delay in answering your questions. We have been swamped with orders today and I did not get a chance to answer. I have never seen a low-pressure sensor from the factory that has a Shrader valve. Most of the time to replace the valve a new one has to be brazed into place. If it was me I would bypass the valve by joining the two wires together that go into the valve so you can use your heat pump. I would not recommend opening up a 25-year-old heat pump to replace a defective low-pressure sensor valve. I would recommend bypassing the sensor and keep a watch to make sure the heat pump does not run low on refrigerant charge. Most of the time the coil’s temperature sensors are held in place by a metal clip and the black gum insulation tape is used as an insulator. Thanks for asking the questions. I hope you have a great day and week! Steve

  9. I can push the contactor in and the fan comes on, but when I release it stops. I replaced contactor and capacitor too

    1. Hi Tommy!
      Sounds like since you can not get the contactor to engage without manually pushing the contacts down that you either have a thermostat problem, thermostat wire problem (possible broken or loose wire), high or low pressure safety switch problem, time delay relay problem (if equipped), low voltage fuse blown problem, if this is a heat pump it could be a defrost control board problem or a low voltage transformer problem. I would suggest that you start testing with a volt meter between R and C you should have 24 to 28 volts. If not you have a transformer or broken wire problem. With the thermostat calling for cooling you should have 24 to 28 volts between the Y and C terminals to energize the outdoor contactor. If you do not have 24 volts between Y and C then you either have a thermostat problem or a broken thermostat wire problem. We have a post about this problem on the following page: https://arnoldservice.com/problem-my-air-conditioners-contactor-will-not-engage-i-am-not-getting-24-volts-to-the-contactor-to-make-it-engage-what-could-be-the-problem/ You would need to keep checking for 24 volts between the Y and C wires. Where you stop getting the 24 volts is where the problem is located. I hope you can easily find and fix the problem.
      Steve

  10. Steve,

    I’ve narrowed down my problem to a bad high pressure switch. Is there a way to replace the switch without draining the system?

    Shawn

    1. Hi Shawn! The only way I know to replace a high pressure switch is by brazing a new one into the refrigeration circuit. I would suggest that you by-pass the switch by joining the two wires together. I would suggest that you install gauges to make sure that the pressure is not actually running high. Problems that would cause high head pressure would be a dirty condensing unit, slow fan motor, over-charge or refrigerant or a blockage in the refrigeration cycle. Most of the times high pressure switches are doing their job by protecting the unit from harm and rarely go out. Steve

  11. Just replaced the motor and capacitor. Fan won’t move, when the unit is turned on it just says “trip”. Not sure what to do next.

    1. Hi Shannon! So sorry to hear that you are having problems with you outdoor AC unit. If the high pressure safety switch is tripped it will not allow the fan to start because the safety switch will not allow 24 volts to go to the contactor to get the compressor and fan motor started. I would suggest that you try to reset the tripped switch, usually by pushing it in with your finger. If the high pressure switch will not reset then you either have a bad high pressure switch or you have a blockage in your refrigeration system that is causing high pressure in your system. I would suggest that connect some gauges to your unit to see what the pressure is actually doing in your system. You should be able to temporarily wire around the pressure switch by turning the power off to the unit and joining the two wires together that go into the high pressure switch. I hope that when you say, “trip” that you are referring to the high pressure switch and not the circuit breaker. If you have more questions please let me know by emailing me at arnoldservice@gmail.com email address. I hope you can easily find and fix the problem. Steve

  12. Steve, thank you so much for all of your help and guidance. I couldn’t have narrowed this down without you. I was searching for the wires that were coming from the thermostat to the condenser and uncovered a HUGE problem. Had the inspector seen it before we bought the house, we wouldn’t have made the purchase (with a list of other issues to add). Previous owner/s had covered up the rusted, chewed up, taped up in multiple places, green and burnt colored, muddy and wet thermostat wires. Basically mucked them up real good and then put about 7 band-aids on them within about a 18″ span. Pulled them, reran new, hooked it all back up and wallah! Perfect! Thank you again, Steve !

    1. Fantastic! Glad to hear that you found and fixed the problem with little expense! I appreciate your kind words and letting us know what the problem was! God bless you and your family today and always! Steve

  13. Steve, we narrowed it down to the culprit! The wires that are coming from the thermostat to the contact, were chewed to all get out. I pulled back the blue flex-hose that was protecting the thermostat wires from the condenser to the house, and found the wires and been chewed up by what appeared to look like a weed-eater. Wires were rusted, taped in about 7 different places, muddy and wet, green colored exposed wires etc. just NASTY looking and came apart with one gentle tug to get it out of the house wall a little bit. Surprised they hadn’t shorted out or burned anything up, or down. Bought new wire and air return filter for $15, ran it, connected it all up, and WALLAH! Thank you Steve for all you do and for all of the help!!

    1. Fantastic! Glad to hear that you found and fixed the problem with little expense! I appreciate your kind words and letting us know what the problem was! God bless you and your family today and always! Steve

  14. my condenser unit isn’t running. The inside unit is blowing air. I check outside at the contactor and it is in the disengaged position (closed circuit). I can manually turn the unit on if the bridge/switch is held down, but it shuts off as soon as i let up. I have 220 voltage on one side coming in, but none coming out unless I hold that bridge down. Could this be a high or low pressure sending unit and can I as a home owner do it myself? I dont make much money so I try to pinch everywhere I can. If I have to call someone out I can, but I try not to. Thank you for your reply and help!

    1. Hi Jerry! I would suggest checking to see if you are getting 24 to 28 volts AC to your contactor coil. If you are not getting the 24 volts to the coil then we cover this problem on the following page: https://arnoldservice.com/problem-my-air-conditioners-contactor-will-not-engage-i-am-not-getting-24-volts-to-the-contactor-to-make-it-engage-what-could-be-the-problem/ If you are getting 24 volts to the contactor coil and it isn’t engaging then you have a contactor problem and you need a new contactor. We sell many different contactors on the following page: https://arnoldservice.com/product-category/air-conditioner-parts/ac-contactors-heat-pump-contactors/ Yes, this could very well be a low pressure (low refrigerant) switch that is cutting the unit off for protection. I hope you can easily find and fix the problem. Steve

      1. Thank you Steve! Now…how to check that darn 24V. lol. I know (i suppose) I have 248 coming in from the power source(condensers own small breaker box which is mounted to the house directly beside the condenser) to the conductor switch, but Im not sure which of the remaining 6 prongs to check for the other side of the contactor 🙁 2 prongs seperate the bridge, or the place where contact opens the circuit. The other 4 remaining prongs are at the oposite end of the 220 side. I wish I could upload a picture of what Im looking at for you guys. I’m so ignorant with HVACs. I know its 248 “coming in” because there are three wires, white, black, and a gnd. I turn my multi-meter to ac, A~, and V and the leads on the white and common/black wire and it spits out 248+-. I move to the top of the contactor and I have 4 wires with 4 prongs, all but right up against each other. Thank you again so much for your help, Steve!

      2. Hi Jerry. Please see the video that we have on the following page: https://arnoldservice.com/problem-my-air-conditioners-contactor-will-not-engage-i-am-not-getting-24-volts-to-the-contactor-to-make-it-engage-what-could-be-the-problem/ It shows how to test the contactor coil to make sure it is getting voltage. I hope you can get the problem fixed soon. Steve

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