How to Test a Varistor With a Multimeter

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The person typing how to test a varistor with a multimeter wants to know whether a surge-protection part on a circuit board, in a power strip or in an appliance has failed. A varistor, usually a metal oxide varistor (MOV), should look almost like an open circuit at normal voltage. A multimeter resistance check can show a part that has become a short, and it can show a part that is clearly out of its normal state. It cannot prove that the varistor will clamp a surge, and that gap matters. This guide explains the check, what the Littelfuse notes say about the high-resistance state, and what the check cannot show.

Quick answer

• Unplug the device, then discharge and isolate the varistor. Measure its two leads with the meter on resistance.
• Littelfuse describes a varistor at low current as a high-resistance element that “appears as an open circuit”, so a healthy part should read high, not near zero.
• A near-zero reading suggests a conducting part. The sources we opened do not give a number that separates good from bad.
• A normal-looking reading does not prove the varistor can clamp a surge. Replace a varistor that looks damaged or is in doubt, using the part number from the device’s own documents.

Electrical safety first

Varistors sit across mains lines in power strips, chargers, appliances and power supplies. Work on mains circuits can kill. Unplug the device, and do not test a varistor while the device is plugged in or while any part of the circuit is energised. If the varistor is inside a sealed appliance or a panel, and you are not sure how to isolate it, stop and call a qualified technician.

Fluke’s general resistance guide gives the basic rule for any component: turn off power to the circuit, and discharge any capacitor in the circuit before you take a resistance reading. A varistor sits in parallel with the line, and a power supply often has capacitors close by, so this rule applies with care. Discharge the device’s bulk capacitors by the method the device’s manual gives, before you probe the board.

The CAT rating of your meter must suit the circuit you are working near. This guide’s resistance check is made on a dead, unplugged device, but a meter used around mains should still carry a rating that suits the installation. Check the rating on the meter’s label or manual, and do not assume it.

What you need

• A digital multimeter with an ohms range that reads from a few ohms into the megaohm range.
• Test leads in good condition, with the black lead in COM and the red lead in the VΩ jack.
• The varistor’s part marking, or the device’s service documents, so you can read the part’s voltage rating if it is marked.
• A small tool to lift one lead of the varistor from the board, if it is soldered in circuit.

For the meter, our best multimeter guide lists the features to compare. If you are checking a circuit for a fault before you reach the varistor, the continuity tester guide explains how to read a beep and an OL display.

Step by step

1. Unplug and discharge. Disconnect the device from the supply. Discharge the device’s capacitors by the method its manual gives, then confirm that no stored charge is left before you touch the board.
2. Isolate the varistor if you can. Fluke says the best results come when the component is removed from the circuit, because the other components in parallel affect the reading. On a board, you may need to lift one lead of the varistor so that the meter reads the varistor alone. Fluke also warns that a meter reading is the total resistance through all possible paths between the probes.
3. Set the meter to ohms. Turn the dial to resistance. Fluke notes that ohms often shares a dial spot with continuity, capacitance and diode tests, so check that the display shows a resistance unit or OL and not a beep symbol.
4. Check the open-lead display. Fluke says the display should show OL before the leads touch anything, because the meter is measuring as soon as it is in resistance mode.
5. Measure across the two leads. Touch one probe to each lead of the varistor. Hold the probes by their insulated bodies. Fluke says body contact with the metal ends adds a parallel path through the body and lowers the reading.
6. Read and record the number. Note the value and its unit, and whether the display shows OL. Then remove the probes in reverse order.
7. Compare with the expected state. Use the high-resistance description in the next section, not a figure from a random website, to judge the reading.

What a healthy varistor should read

The Littelfuse application note describes the varistor’s behaviour across its operating range. It says the varistor is “voltage dependent, nonlinear”, and that when exposed to high voltage transients its impedance “changes many orders of magnitude from a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level.” That is the surge behaviour. At normal voltage, below the level where it conducts, the note describes a different state.

In the low-current, or leakage, region, the note says that “the V-I Curve approaches a linear (ohmic) relationship and shows a significant temperature dependence,” and that the varistor “is in a high resistance mode” and “appears as an open circuit.” So a healthy varistor on the bench should read as a very high resistance, close to an open circuit, not as a short. The note gives the high-resistance value only as a superscript that our copy does not render reliably, so this guide does not quote a number and uses the description instead.

Use the reading in three steps:

• High or OL: consistent with the high-resistance state the note describes. This is a screening result. It does not prove the varistor will clamp a surge.
• Near zero: the part is conducting, which is not the high-resistance state. Check the leads, the board and any parallel path before you draw a conclusion, because a parallel path can lower a reading.
• A number between the two: the sources we opened do not give a pass or fail value for a varistor resistance check, so do not decide from a single middle reading. Compare it with the part’s datasheet for leakage current at its working voltage.

Leakage current and why one number is not enough

Littelfuse’s HMOV datasheet lists a maximum leakage current for its parts, and it gives the coating insulation resistance as a separate figure. A datasheet leakage limit is set at a stated test voltage, so a multimeter test at a different voltage is not the same test. The multimeter applies its own small test voltage, and the varistor’s resistance depends on the voltage across it. The Littelfuse note also says the leakage characteristic changes with temperature, which means a hot varistor and a cold one can read differently.

That is why a multimeter check is a screening step. It can find a varistor that has clearly become a short. It cannot tell you whether the part will clamp a surge at its rated voltage, and it cannot tell you the part’s remaining life after past surges.

Signs to check before you measure

Inspect the varistor by eye before you test it. Look for cracks in the ceramic body, a dark or burnt area, or a coating that has split. The sources we opened do not give a visual pass or fail rule, so treat what you see as a reason to replace the part, not as proof of how it will perform. A part that has visible damage should not be put back into service, whatever the meter says.

Mistakes to avoid

1. Testing a varistor while the device is plugged in. Unplug it first.
2. Skipping the discharge. Capacitors near the varistor can hold charge after unplugging.
3. Measuring in circuit without a schematic. Fluke says parallel paths change the reading, usually lowering it, so check the schematic first.
4. Reading a near-zero result as a fail without checking the board. A parallel path can make a good part read low.
5. Holding the probe tips. Body contact lowers the reading.
6. Treating a normal reading as a safety certificate. The check does not test surge clamping.

When to stop and get help

Stop and call a qualified technician if the varistor is inside a panel, a service entrance device or any equipment you are not sure how to isolate. Replace a damaged varistor with the same part number and voltage rating from the device’s documents. Do not substitute a part of a different rating because it fits the board. A device that has been hit by a lightning strike, a surge or a fault should be checked as a whole, not only at the varistor.

How we chose

We chose the steps in this guide by asking whether a manufacturer’s own application note or datasheet states them in plain words. We opened the Littelfuse application note AN9767.1 on varistor basics, in a copy hosted by Galco. We opened the Littelfuse HMOV radial leaded varistor datasheet, in a copy hosted by TTI, for its leakage and insulation specification entries. We opened the Fluke guide to measuring resistance for the general meter steps. We did not use blog posts, forum threads or distributor listings to set any step.

What we could not verify

The sources we opened do not give a multimeter test procedure for a varistor, a pass or fail resistance value, or the test voltage a typical meter applies. The Littelfuse datasheet’s leakage limit is set at a stated test voltage, and we did not find that voltage for a meter test. The sources do not say how a varistor’s resistance reading changes with the meter’s own test voltage in circuit. They do not give a surge-clamping check that a meter can perform. For the rating of a specific part, use its own datasheet.

FAQ

What does a good varistor read on a multimeter?

The Littelfuse note describes a high-resistance state that “appears as an open circuit” at low current. A healthy part should read high, not near zero. The sources we opened give no pass or fail number.

Can I test a varistor in circuit?

It is better out of circuit. Fluke says parallel components affect a resistance reading, so check the schematic and lift one lead if you can.

Does a normal reading mean the varistor still works?

Not necessarily. The check screens for a shorted part. It does not test surge clamping, which the datasheet covers at stated test conditions.

Why does the reading change?

Littelfuse says the leakage region is strongly temperature dependent, and the varistor’s resistance depends on the voltage across it. A hot part and a cold part can read differently.

Should I replace a varistor that reads near zero?

Check the board for a parallel path first. If the part is conducting, replace it with the same part number and voltage rating from the device’s documents.

Bottom line

Unplug, discharge and isolate the varistor, then measure it on resistance. A healthy part should look like an open circuit at normal voltage. A near-zero reading is a warning, but a normal reading does not prove the part will protect the circuit.

Sources

• https://pim.galco.com/Manufacturer/Littelfuse/TechDocument/Product%20Information%20Sheet/lf_mov_inf.pdf (Littelfuse, Varistors – Basic Properties, Terminology and Theory, AN9767.1, copy hosted by Galco; opened 2026-10-27)
• https://www.tti.com/content/dam/tti-commons/supplier/littelfuse/doc/littelfuse-hmov-series-radial-leaded-varistors-datasheet-specifications.pdf (Littelfuse, HMOV Varistor Series datasheet, copy hosted by TTI; opened 2026-10-27)
• https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (Fluke, How to Measure Resistance with a Digital Multimeter; opened 2026-10-27)