What a pressure switch does and why it might stop working
A pressure switch is a small device that turns something on or off when pressure inside a system reaches a certain level. You'll find them in washing machines, air compressors, water pumps, HVAC systems, and dishwashers. When the switch fails, the appliance either won't start, won't stop, or cycles incorrectly.
The switch itself is usually a metal cylinder about the size of a AA battery, mounted on the side or top of the tank or system. Inside is a spring and a metal contact that closes or opens an electrical circuit. Over time, the spring weakens, the contacts corrode, or the diaphragm (a thin rubber seal) cracks, and the switch stops responding to pressure changes.
Before you replace it, you can test whether the switch is actually the problem. This takes a multimeter, a few minutes, and no special tools.
Key Takeaways
- A pressure switch controls when an appliance or system turns on and off based on internal pressure levels.
- You can test a pressure switch with a multimeter by checking for electrical continuity when pressure is applied and released.
- The switch should show a closed circuit (continuity) at high pressure and an open circuit (no continuity) at low pressure, or vice versa depending on the type.
- If the switch shows no change in continuity when pressure changes, it has failed and needs replacement.
- Always turn off power to the system and release all pressure before disconnecting the switch.
Turn off power and release pressure safely
Before you touch anything, cut power to the system at the breaker or unplug it. For air compressors, water systems, and HVAC equipment, also open any drain valve or release valve to let all pressure out of the tank. Wait a few seconds and listen — you should hear air or water escape, then silence.
If you're not sure where the release valve is, check the manual or look for a small lever or button on the tank. On washing machines and dishwashers, you usually just need to unplug the unit. On HVAC systems, turn off the thermostat and the breaker.
This step is not optional. Pressurized systems can cause serious injury if you disconnect a live switch.
Locate the pressure switch and identify its terminals
Find the switch on your system — it's usually a small metal cylinder with two or three wire terminals sticking out of the bottom or side. Take a photo of how the wires connect before you disconnect anything. You'll need to know which wire goes where when you reconnect it.
Most pressure switches have two terminals (for a simple on/off function) or three terminals (for switches that control multiple circuits). Write down or photograph the wire colors and which terminal each one connects to. If the wires are labeled, even better — note those labels too.
If the switch is glued or sealed to the tank, do not pry it off. You can test it while it's still connected by checking the terminals with a multimeter.
Test continuity with a multimeter at rest
Set your multimeter to the continuity or resistance setting (usually marked with a sound-wave symbol or the Greek letter omega). Touch one probe to each terminal of the switch. A multimeter with continuity mode will beep if electricity can flow between the terminals; a resistance meter will show a very low number (close to zero ohms) for continuity or a very high number (often displayed as "1" or "OL") for no continuity.
Write down what you see. For a two-terminal switch at rest (no pressure), you should see either continuity or no continuity — the switch is in one state or the other. For a three-terminal switch, test each pair of terminals and note which ones show continuity.
This is your baseline. The switch should change state when pressure is applied.
Apply pressure and test continuity again
Now you need to trigger the switch by applying pressure to the system. For an air compressor, turn the power back on and let it build pressure — the compressor will run until it reaches its normal operating pressure, then the switch should trigger and the motor should stop. While the compressor is running and building pressure, test the switch terminals again with your multimeter.
For a water system or pump, turn the power back on and let the pump run until it reaches pressure. For a washing machine or dishwasher, you may need to start a cycle and catch it at the moment the pump is running. For HVAC systems, set the thermostat to heat or cool and let the system run.
The key is to test the switch while the system is under pressure. The continuity reading should change from what you saw at rest. If it was showing continuity at rest, it should show no continuity under pressure, or vice versa.
Interpret your results
A working pressure switch will show a change in continuity between the rest state and the pressurized state. If you saw continuity at rest and no continuity under pressure, that's normal for a normally-closed switch. If you saw no continuity at rest and continuity under pressure, that's normal for a normally-open switch. Either way, the switch is responding to pressure.
If the continuity reading does not change when you apply and release pressure, the switch has failed. The contacts are stuck, the diaphragm is ruptured, or the spring is broken. The switch needs to be replaced.
If you see continuity in both states or no continuity in both states, the switch is also bad. A working switch must change state when pressure changes.
What to do if the switch has failed
Write down the brand, model number, and pressure rating of the switch before you order a replacement. This information is usually printed on the metal cylinder itself. You can also check your system's manual or search online for the part number.
Pressure switches are not universal — a switch rated for 40 PSI will not work correctly on a system that runs at 100 PSI. Order the exact replacement for your system. Once it arrives, disconnect the old switch, connect the new one using your photo as a guide, and test it the same way you tested the old one.
If the new switch fails after a few weeks, the problem may not be the switch itself — it could be a leak, a failing pump, or a stuck valve that's preventing the system from reaching the correct pressure. In that case, you may need to call a technician to diagnose the underlying issue.
Frequently Asked Questions
Can I test a pressure switch without a multimeter?
Not reliably. You can listen for a click when pressure builds (some switches make an audible sound), but you cannot confirm electrical continuity without a multimeter. A multimeter costs $10 to $30 and is worth having for any appliance troubleshooting.
What if the switch has three terminals instead of two?
Test each pair of terminals separately. One pair may control the motor, another may control a safety circuit. Write down which pairs show continuity at rest and which change when pressure is applied. Compare your results to the wiring diagram in your system's manual to determine if the switch is working correctly.
Do I need to replace the switch if it clicks but the system still doesn't work?
Not necessarily. If the switch is responding to pressure (you hear it click and the multimeter shows continuity change), the problem is elsewhere — a broken wire, a failed motor, a tripped breaker, or a stuck valve. Test the wires and other components before replacing the switch.
Can a pressure switch be repaired instead of replaced?
In rare cases, you can clean corroded contacts with fine sandpaper or electrical contact cleaner, but this is temporary. Once a switch fails, replacement is the standard fix. Rebuilt or refurbished switches are sometimes available but are not much cheaper than new ones.
How do I know what pressure the switch should trigger at?
The pressure rating is printed on the switch body or listed in your system's manual. Common ratings are 30 PSI for water systems, 40 PSI for small air compressors, and 90 to 120 PSI for larger compressors. Do not guess — order the exact replacement for your system.