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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.
I've shown many clients how to check their low pressure switch, so here, I’ll you with a simplified step-by-step guide to accurately perform these tests in a matter of minutes.
ACLAB note:
A low pressure switch should only be checked by those who are comfortable with minor electric work. If you at unsure at any step, please stop and call a professional HVAC technician.
Testing a low-pressure switch (Step-by-step guide)
Step 1: Disconnect the power
Take out the device’s power by either unplugging or turning off the circuit 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.
I highly recommend taking a picture of the wires before you disconnect them, so you know how they should be returned.
Step 4: Check the resistance using a multimeter
Now that you’ve isolated the leads from the switch, let's check continuity by turning 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 by giving you an ohms reading of 1 or more, or displaying infinity, then there’s a short or an open in the circuit.
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 unit to figure out the numbers.
ACLAB note:
If you want to double check the continuity, you can change your multimeter resistance setting to the electronic beep labeled as a soundwave or 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).
No “beep” or no reading on your meter means you have an open circuit or “infinity.”
If there’s no continuity, you won’t hear the beep, which means the switch is open and should be replaced. I highly recommend to look at the Ohms reading while doing this and to not rely on the “beep” sound from the meter.
Step 5: 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
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.
These "short cycles" not only leave us uncomfortable, but also significantly increase energy consumption of the unit.
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.
Related article: Understanding AC short cycling and how to fix it
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 or high-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.
Related article: 10 Reasons your AC isn't turning on
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.
If your house is getting hot and you notice there is no air coming out of the registers or from the evaporator unit there are a couple things to do before you have to call in a technician.
First, check your thermostat and ensure its set on “cool” and the temperature setting is lower than the room temperature.
Next, turn your fan setting from ‘Auto” to “on.” This will at least tell you if the indoor fan is working.
If the outdoor unit does not turn on then it may be time to call the service technician.
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 a licensed HVAC technician with a Section 608 Technician Certification.
If you’re not certified, you will want to reach out to a professional to do this for you.
Clear the refrigerant
Refrigerants are very expensive. The refrigerant you take out should be put back in to save you anywhere from $100 to $500.
To start with, take out the existing refrigerant from the system. In doing so, don’t forget to adhere to environmentally-conscious steps to handle such types of gases.
Discharging the refrigerant into the air isn’t allowed as it adds to the overall greenhouse gases.
In accordance with the Clean Air Act (CAA), the federal government can impose fines to people and businesses that exceed $37,000 per day for the mishandling of refrigerants. Depending on the severity, this can even include criminal charges and jail time.
Replace the switch
Once you find the low-pressure switch, just follow the steps below:
Remove non-condensables
Make sure you remove the water, air, contaminants and any other non-condensables entirely from the AC unit before filling it again with refrigerant. Here’s how to do it:
Related article: Best HVAC vacuum pumps for moisture removal
Restore the refrigerant
Once you’ve dried out the unit entirely, you’ll have to shut service valves and disengage the HVAC vacuum pump.
Related article: Top-rated HVAC gauges recommended by professionals
How to find the low-pressure switch (For beginners)
There are three low pressure switches on your HVAC system, including the furnace.
- 1Furnace low pressure vent switch:
- First, open your furnace using an appropriate-sized nut driver or screwdriver to expose the burner assembly.
- If you inspect carefully, you'll see a thing that seems like the snail connected to the discharge. It's a draft inducer.
- Off that configuration, you’ll find one (possibly two) ¼ inch hoses leading to the HVAC low-pressure switch. - 2Low pressure switch on the furnace gas valve train:
- This switch is located near the burner to ensure correct gas pressure before entering the burner assembly. - 3Low pressure switch on the air conditioner refrigerant system:
- 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.
FAQs
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?
Bypassing a low-pressure switch in HVAC involves temporarily connecting its terminals, but this is unsafe and should only be done by a professional for diagnostic purposes. It’s a critical safety feature, and bypassing it long-term can damage the system or pose hazards.
