Testing Low-Pressure Switch (Beginners Guide To HVAC)

Josh Mitchell

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Josh Mitchell

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Holly Curell

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Even if you suspect there’s a problem with your HVAC low-pressure switch – you don’t have to call a professional to check it out.

As I’ve done this numerous times, I’ve decided to provide you with a simplified step-by-step guide to accurately perform these tests in a matter of minutes.

Testing A Low-Pressure Switch (Step-By-Step Guide)

Here’s how you can test a low-pressure switch in only a few minutes.

Step 1: Disconnect The Power

First thing first, take out the device’s power by either unplugging or turning off the electric breaker supplying the power.

Step 2: Locate The Low-Pressure Switch

Typically, you will find the low-pressure switch on the suction line near the compressor or inside the compressor housing.

It will look like a small, rounded component with two wires connected to it, often secured to the line with a threaded connection or a snap-fit.

Step 3: Remove The Wires

Now, trace the wires from the switch to the terminal block and separate them from the rest.

The wires (also called leads) coming out of the switch should be blue or brown, but I’ve seen them in yellow, so they should be easily distinguishable.

They should also be fairly easy to disconnect, so just apply a little bit of pressure and pull them out.

Step 4: Check The Resistance Using A Multimeter

Now that you’ve isolated the leads from the switch, turn your multimeter to the resistance (Ohms, labeled as Ω) setting, to test the resistance in an electrical circuit.

Do this by touching one probe to each wire. Make sure the probes have good contact with the metal part of the wire connectors. If necessary, you may remove the plastic plugs to expose the metal part of the leads for better contact.

If the multimeter reads around 0, your low-pressure switch is working. If the readings are off, there’s a shortage in the system.

Any reading below 0.5 is good, and the higher you go, the worse the results are.

However, on some devices, the resistance could be between 1 and 10, in which case, I’d consult the manual you got with the AC to figure out the numbers.

Step 5: Check For Continuity Using A Multimeter

Change your multimeter setting to check for continuity, which should be right below the resistance setting on the dial, labeled as a soundwave or a triangle with a line on the right side.

Again, touch the probes to the switch’s leads. 

If the switch is closed (indicating it works properly), the multimeter will complete the circuit between the two wires, resulting in a continuity signal (you’ll hear a continuous “beep” sound or a see specific reading on the display).

If there’s no continuity, you won’t hear the beep, which means the switch is open and should be replaced.

Step 6: Reattach And Power On

After testing, reconnect the wires to the switch and ensure they are securely attached to the correct terminals.

Once that is done, power up the system and observe how the AC is operating to ensure everything is functioning correctly.

Signs Of Bad Pressure Switch

Portable AC Unit

Here are a few tell-tale signs when you should test your HVAC low-pressure switch:

Intermittent Air Conditioning

Intermittent air conditioning, or simply AC cycling is the most common issue I see with faulty low-pressure switches.

If the low-pressure switch is faulty, it will “misread” the refrigerant pressure level, causing the compressor to shut off, resulting in an “on and off” workflow, leaving you neither warm nor cool.

What’s also wrong with this is that it will put unnecessary strain on your compressor, effectively lowering its lifespan, which is the exact opposite of what a low-pressure switch should do.

Air Conditioning Stops Working

If your air conditioning system is not turning on at all, it may not be the compressor that’s at fault!

Chances are, the refrigerant low-pressure switch sensor could be defective and is preventing your unit from working.

Although I must say, in my experience, this isn’t the case that often. If it comes this far, it is more likely that there is a bigger issue at hand.

Warm Air Is Blowing

If instead of throwing cold air, the AC pours out warm air, there’s a solid chance your AC’s low-pressure switch is busted open.

Then again, it is not uncommon for the low refrigerant to also trigger this issue, so check that first, before you pull out the wires to check the switch or call an HVAC guy.

Strange Air Conditioning System Noises

If the AC compressor clicks on and off continuously, it will generate some unusual sounds. You’ll hear the distinct clicking sound whenever the AC compressor starts up, even if it doesn’t actually kick into gear later on.

So, always listen to your AC and you should be able to tell if the pressure switch turns the compressor on and off.

How To Easily Replace A Low-Pressure Switch

Here’s your ultimate step-by-step guide to replacing your HVAC low-pressure switch.

Beware, though, that to purchase or handle the various refrigerants in the US, you must be certified. If you’re not certified, you will want to reach out to a professional to do this for you.

Clear The Refrigerant

To start with, take out the existing refrigerant from the system. But, in doing so, don’t forget to adhere to environmentally-conscious steps to handle such types of gasses.

Discharging the refrigerant in the air isn’t allowed as it adds to the overall greenhouse gasses.

You’ll have to attach the hoses of a recovery machine with the AC valves so that it discards the refrigerant safely.

Replace The Switch

Once you find the low-pressure switch, just follow the steps below:

  • Unscrew the pressure switch from the position using your screwdriver.
  • Disconnect the wires and remove the switch.
  • Now, replace the existing rubber ring with the newly purchased one.
  • Insert your new HVAC pressure switch.
Unscrewing an HVAC AC unit

Remove The Moisture

Make sure you remove the humidity entirely from the AC unit before filling it again with refrigerant. Here’s how to do it:

  • Set the AC gauge and connect it with pressure ports aptly detailed in one of the first steps of this procedure.
  • Connect the HVAC vacuum pump from a gauge and activate it along with the service valves.

Restore The Refrigerant

Once you’ve dried out the unit entirely, you’ll have to shut service valves and disengage the HVAC vacuum pump.

  • Fit in the can available with a gauge set.
  • Now attach a refrigerant can to it.
  • After that, you’ll need to open up the can along with service valves on a gauge set, providing the unit with a new refrigerant.
  • Keep repeating the process till the unit gets full. Grab your manufacturer’s manual and see how much is enough, and that’s all.

How To Find The Low-Pressure Switch (For Beginners)

If you are looking for even clearer instructions on how to find the HVAC low-pressure switch on your unit? It’s easier than you think. Here’s what you need to do:

  1. 1
    First, open your furnace using an appropriate-size nut driver or screwdriver to expose the burner assembly.
  2. 2
    If you inspect carefully, you’ll see a thing that seems like the snail connected to discharge. It’s a draft inducer.
  3. 3
    Off that configuration, you’ll find one (possibly two) ¼ inch hoses leading to the HVAC low-pressure switch.

People Also Ask (FAQs)

How Do I Test An AC Pressure Switch With A Multimeter?

To test an AC pressure with a multimeter, first you’ll:

  1. 1
    Place a multimeter in an Ohms position.
  2. 2
    Locate the terminals.
  3. 3
    Touch each terminal with your leads. Remember, the closed pressure switch reads ‘0’ ohms; however, if it provides an ‘infinity’ reading – you have a faulty pressure switch.
How Many Ohms Should A Pressure Switch Be?

The multimeter should show a ‘0’ reading when the pressure switch is fully functional (closed). If it doesn’t close, the ohmmeter will give an ‘OL’ or ‘infinity’ reading.

Related Article: How to Read HVAC Gauges

How Does A 40 60 Pressure Switch Work?

The 40/60 pressure switch works by signaling the pump to activate when the pressure drops to 40 PSI and keep it working until the pressure reaches 60 PSI, when it will signal the pump to turn off.

What Causes A Pressure Switch To Be Stuck Closed?

There can be several things causing a pressure switch to be stuck closed, such as damaged or faulty inducer and wheel blockage in vents, among other things.

How Do You Bypass A Pressure Switch?

Here’s how to bypass a pressure switch:

  1. 1
    Shut off the unit along with the breaker providing power.
  2. 2
    Remove the nuts that are holding the service cover using your wrench.
  3. 3
    Unclip the wires from the metal contacts.
  4. 4
    Now, join the connectors using electrical tape, and that’s all.


As you can see, troubleshooting and testing the low-pressure switch is an easy task you can accomplish in mere minutes even if you’ve never done it before – provided you have the right tools, such as a multimeter.

Now that you know how to do this, the next time you feel the warm air or hear a clicking sound coming from your AC, you’ll know exactly what to look for.

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Josh Mitchell


Josh Mitchell
My name is Josh and I am obsessed with home appliances. From portable AC units to heaters and air purifiers, I enjoy testing, learning and using these devices to improve the air quality inside my family home.

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Midea U Shaped Window Air Conditioner

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