The pressure switch tells the motor when to start and stop. When the contacts fail, the motor gets no power -- or the switch gets stuck and won't cycle. Usually a straightforward replacement at $20-$60.

What you'll see

The compressor does absolutely nothing. No hum, no click, no attempt to start, even though there's power at the outlet and the tank is empty. Or the switch clicks but there's no power to the motor. Sometimes the contacts are burnt and make poor connection, so the motor starts intermittently or buzzes weakly.

The pressure gauge on the switch may work fine, the switch just doesn't close the circuit. On some switches, you can see the contacts are blackened or pitted when you remove the cover.
Before you assume this is the problem

First check: is there actually power at the outlet? Plug in a lamp or tester. Check the breaker and any GFCI outlets. If the motor hums but doesn't spin, the switch is working but the problem is elsewhere (capacitor, mechanical). See: Bad Start Capacitor, Unloader Valve.

See all causes of compressor won't start →

How to diagnose

  1. Check for power at the switch input terminals

    With the cover off and a multimeter, check voltage at the input (line) terminals. You should see full supply voltage (120V or 240V depending on your compressor). No voltage here means the problem is upstream -- breaker, cord, or outlet.

    Result: Full voltage at input = power supply OK. No voltage = check breaker, cord, outlet.

  2. Check for power at the switch output terminals

    With the tank below cut-in pressure (the switch should be calling for the compressor to run), check voltage at the output (load) terminals. If you have voltage at the input but not at the output, the switch contacts aren't closing. Try tapping the switch gently -- if it starts, the contacts are burnt and making poor connection.

    Result: Voltage at output = switch contacts OK. No voltage at output = contacts failed, replace switch.

  3. Inspect the contacts visually

    Remove the switch cover and look at the contacts. Burnt, pitted, or blackened contacts are worn out. You may see carbon deposits bridging the contacts. On some switches you can clean the contacts with fine sandpaper as a temporary fix, but replacement is the proper solution.

    Result: Clean, shiny contacts = OK. Burnt, pitted, or blackened = replace.

How to fix it

  1. Replace the pressure switch

    Match the replacement switch to your compressor's voltage, amperage, and pressure range. Most small shop compressors use a standard 95-125 PSI switch (cut-in at 95, cut-out at 125). Note the wiring before disconnecting -- take a photo. The new switch usually comes with the unloader mechanism attached. Apply thread sealant to the pressure port fitting.

  2. Set the pressure correctly

    If the new switch is adjustable, set the cut-in and cut-out pressures to match your original settings. The big spring adjusts the cut-in pressure, the small spring adjusts the differential (the gap between cut-in and cut-out). Don't set the cut-out pressure higher than the tank rating -- check the tank nameplate.

Common mistakes

Don't bypass the pressure switch with a direct connection -- that removes the safety shutoff and the compressor will run until the safety relief valve pops or something fails catastrophically. Also: when buying a replacement, match the voltage and amperage rating. A switch rated for 120V on a 240V compressor will arc and burn out fast. If you're getting switches that fail repeatedly, the motor may be drawing too much current -- check for other problems causing motor overload.

Parts & tools

Replacement pressure switch (match voltage, amperage, pressure range). Multimeter. Screwdriver for terminal connections. Thread sealant tape. Phone camera for wiring photo before disconnecting.

Review safety precautions before starting →

Safety

The pressure switch carries full motor voltage -- typically 240V for larger compressors. Disconnect power at the breaker before working on or removing the pressure switch. Don't just turn off the compressor switch, pull the plug or flip the breaker.