HOW-TO GUIDE
How to Test a Microswitch With a Multimeter
By the OhmSavvy Research Team ·
Unplug first, and treat microwaves with extra care
- Appliance microswitches switch mains: 230 V in the UK and Europe, 120 V in North America. Unplug the appliance at the socket before removing any cover; switching it off is not enough.
- A microwave's high-voltage capacitor can hold a charge of several thousand volts after the oven is unplugged, enough to kill. If you are not trained to discharge it safely, do not open the outer case. Interlock switches are reachable, but only after that capacitor is dealt with.
- Never bypass or bridge a door interlock to "test" an appliance with power on.
Where appliance microswitches hide
A microswitch is a small snap-action switch: a light push trips an internal spring that flips the contacts quickly, however slowly you press. Doors, lids and levers use them to report position.
Microwave door
Two or three interlocks, plus a monitor switch, decide whether the magnetron may run.
Kettle
A switch in the base or handle, flipped by the lever and released by the thermostat at boiling.
Washing machine
The door lock assembly includes a switch that tells the board the door is shut and latched.
Mouse buttons
Tiny three-pin switches under each button; worn ones cause unwanted double-clicks.
Lever and roller
Longer actuators reach doors and cams in tumble dryers, dishwashers and vending gear.
Pin plunger
The bare switch, pressed directly by a molded tab or button cap.
1Identify COM, NO and NC on the switch
Most microswitches have three spade or solder tabs. COM is the common contact, NO (normally open) connects to COM only while the actuator is pressed, and NC (normally closed) connects at rest and breaks when pressed. The letters are usually molded beside each tab, sometimes as C, 1, 2 and 4 on European parts. Some appliance switches have only two tabs, so they are either NO or NC.
Take at least one wire off first, or other wiring can create a parallel path and a false beep. New to continuity mode? See how to use a multimeter covers the settings.
2Test continuity with the actuator pressed and released
Set the meter to continuity. With the lever untouched, test COM to NC, then COM to NO. Press the actuator fully, hold it, and repeat both. Press by hand on the lever or plunger rather than relying on the door or cam, so you know the switch is actually being operated.
| Probes on | Actuator released | Actuator pressed |
|---|---|---|
| COM and NC | Beep, near 0 Ω | OL (open) |
| COM and NO | OL (open) | Beep, near 0 Ω |
| NO and NC | OL | OL |
NO to NC should never connect; if it does, the switch is internally damaged. OL simply means the resistance is above the meter's range, which is exactly what an open contact should show. See what OL means on a multimeter for how each mode displays it.
3Measure contact resistance in ohms
A beep only proves resistance is below the meter's continuity threshold, which on many meters is tens of ohms. For a switch carrying a kettle element or a motor, that is not good enough. Switch to the lowest ohms range, touch the probe tips together and note the lead resistance, then measure the closed contact and subtract it.
The click is not the test
Testing microwave door interlock switches
Microwaves typically use a primary and a secondary interlock that close when the door shuts, plus a monitor switch wired across the supply that closes when the door opens. If a primary switch welds shut, opening the door lets the monitor switch short the line and blow the fuse on purpose, so a repeatedly blown fuse is a strong hint that an interlock has failed.
With the oven unplugged and the capacitor safely discharged, remove each switch's connectors and test them one at a time with the door open and shut. Note which tabs each wire came from, as the monitor switch usually uses COM and NC while the others use COM and NO. Fit the exact replacement parts listed for that model. The same reasoning helps with a blown plug fuse on other appliances: our guide to testing a 13 amp plug fuse shows how to confirm it.
Kettles, washing machine door locks and mouse buttons
- Kettles: the switch usually sits in the handle or base with the thermostat. Unplug, let it cool and empty it, then check continuity across the switch with the lever up and down. A kettle that will not stay on may have a worn latch rather than bad contacts, so check the mechanism too.
- Washing machine door locks: the lock assembly often contains a heated bimetal or solenoid as well as the switch. Use the wiring diagram on the assembly or in the service manual to find the switch pair, and don't mistake the heater's resistance reading for the switch contacts.
- Mouse buttons: these switch only a low-voltage logic signal, so the risk is low. A button that double-clicks usually has a worn contact that bounces as it closes. A meter is too slow to see bounce, so a steady reading does not clear the switch; the push button testing guide explains why.
Machine-mounted switches are often tested live in their control circuit instead; see how to test a limit switch for that approach.
Spotting worn or sticking microswitches
| Symptom on the meter | What it usually means |
|---|---|
| Swaps cleanly, under an ohm when closed | Switch is fine; check the actuator, door alignment or wiring |
| COM to NO stays OL when pressed | Contacts burnt or worn, or plunger not traveling far enough |
| COM to NO beeps even when released | Welded contacts, a common microwave interlock fault |
| Closed reading of several ohms, or drifting | Pitted, oxidised or contaminated contacts |
| Beep crackles or cuts out as you press slowly | Worn contact or weak snap spring |
| Plunger stays down or returns slowly | Sticking mechanism, often grease, limescale or heat damage |
A bent lever, flat-spotted roller or cracked door hook can stop a good switch being pressed. If a reading flickers with the actuator held still, check the probe contact before blaming the switch.
Meters for appliance switch testing
A microswitch test needs a quick continuity beeper and an ohms range that reads below one ohm clearly, since contact resistance is what separates a good switch from a pitted one. Appliance work also means confirming the machine is unplugged and any capacitor discharged, which is where a meter with a proper CAT rating earns its place. Compare options in our best multimeter for beginners guide.
Meters for appliance microswitches
Auto-ranging True RMS with a backlit screen that helps inside dim appliance cabinets; CAT II 600V.
Hands-on tested. CAT III 600V and a 6,000-count display, built for repeated appliance and field service calls.
Pocket-sized CAT III 600V meter with NCV; Extech does not list it as True RMS, which doesn't matter for switches.
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Frequently asked questions
Why does my microwave keep blowing its fuse when I open the door?
Often a failed door interlock. Most microwaves have a monitor switch that deliberately shorts the supply if the other interlocks fail to open with the door, which blows the fuse. Replacing only the fuse just repeats it. Unplug, discharge the high-voltage capacitor safely, test every interlock out of circuit, and replace the faulty switch and the fuse together.
Can a microswitch click but not work?
Yes. The click is the spring mechanism snapping over, which can happen even when the contacts are burnt, pitted or no longer touching. Only a continuity or resistance reading proves the circuit actually closes. A switch that clicks normally but reads OL or several ohms across COM and NO when pressed is faulty.
What resistance should a closed microswitch read?
Typically a fraction of an ohm once you subtract your test lead resistance, which is normally around 0.1 to 0.3 Ω on its own. A closed reading that sits at a few ohms, or wanders as you press harder, points to dirty or arc-damaged contacts. For a mains appliance switch that carries real current, that switch should be replaced.
Can I test a mouse microswitch without desoldering it?
Often, yes. Unplug the mouse or remove its battery, open the case and probe the switch pins on the board while clicking. The surrounding circuit can add parallel paths, so an odd reading is not conclusive. Desoldering at least the COM pin gives a clean test, which also lets you check for the erratic contact that causes double-clicks.
Are all three-pin microswitches interchangeable?
No. Match the body size, the actuator type and length, the terminal style, the operating force and the current and voltage rating printed on the body. An appliance switch rated for mains current must be replaced with one carrying at least the same rating, and an interlock should be the part specified for that model.
