How to Check Thermistor with Digital Multimeter
As an Amazon Associate we earn from qualifying purchases.
How to check thermistor with digital multimeter: disconnect the part, set the meter to ohms, and take a reading at room temperature. Then change the part’s temperature and watch which way the number moves. A negative-coefficient (NTC) thermistor reads lower when warmed and higher when cooled. If the reading is open, near zero, frozen, or moves the wrong way, the part is suspect. This guide covers the meter setup, a simple warm-and-cool routine, and how to read the results.
Electrical safety first
Work on mains circuits can kill, and OSHA’s fact sheet on working safely with electricity says that if the ground path is broken, fault current may travel through a worker’s body, causing electrical burns or death. Choose a meter whose CAT rating suits the circuit; Fluke’s page on category ratings says test equipment with a rating that does not match the task can explode if used improperly. Panel and service work belongs to a licensed electrician, and OSHA’s general-industry rule allows only qualified persons to work on parts that have not been de-energized.
Thermistors often sit inside mains appliances such as refrigerators, washers and water heaters. This guide does not cover working inside a powered appliance. It covers a loose thermistor, or a probe that is already out and not connected to any power source, checked in the meter’s ohms function. The CAT rating comes only from the meter’s listing or manual; if the listing does not state one, do not treat the meter as suitable for household mains.
Quick answer
- Isolate the thermistor, set the meter to ohms, and write down the room-temperature reading.
- Warm the part a little and cool it a little. An NTC part falls when warmed and rises when cooled.
- Compare the reading with the part’s datasheet or chart, not with a guess.
- A ceramic PTC thermistor stays low until it nears its switching temperature, so the hand-warm routine does not apply to it.
Know which kind of thermistor you have
Vishay describes NTC temperature sensors as made from a mixture of metal oxides sintered to give a negative resistance versus temperature relationship, so small temperature changes produce a significant change in resistance. The University of Iowa physics demonstration puts the behavior in plain terms: unlike copper wire and most resistors, thermistor resistance goes up as it gets cold and down as it gets hot.
A PTC thermistor has a positive coefficient, but it is not a mirror image of an NTC part. A Vishay datasheet says NTC thermistors sense and measure temperature over a defined range, while PTC thermistors switch at one particular temperature. TDK describes ceramic PTC thermistors as parts whose resistance rapidly increases when a certain temperature is exceeded. The same datasheet limits a PTC sensing part to 250 ohms or less anywhere from −20 degrees C up to 20 degrees below its nominal working temperature, which starts at 70 degrees C, and for nominal temperatures of 80 degrees C and up it requires at least 1,330 ohms 5 degrees above that temperature. That is why a hand-warm test tells you a lot about an NTC part and little about a switching PTC part. If you are not sure which type you have, look for a datasheet or a marking before you decide that a quiet reading means a dead part. Our guide to testing a thermocouple with a multimeter covers a different temperature sensor that is easy to confuse with a thermistor.
Set up the meter
Fluke’s guide to measuring resistance gives the steps. Turn off power to the circuit, and if it includes a capacitor, discharge the capacitor before taking any resistance reading. Turn the dial to ohms. The display should show OL until the leads touch the part. Best results come when the component is removed from the circuit, because components in parallel affect the reading, and Fluke notes the human body becomes a parallel resistance path, so keep fingers off the metal tips of the leads. Clip the leads to the thermistor wires if you can.
Press the range button to hold one range and note the unit on the display, such as k for kilohms. Fluke describes a hold button that captures a stable reading and a MIN/MAX button that captures the lowest and highest measurement. MIN/MAX is useful here, because a warming NTC part gives a falling reading and the recorded highest and lowest values show the full swing. Lead resistance matters only for low values. Fluke puts it at typically 0.2 to 0.5 ohms and the Fluke 87 manual at 0.1 to 0.2 ohms, which is negligible against 10,000 ohms, but the Vishay NTCLE100E3 series starts at 3.3 ohms, where the relative mode earns its place.
Keep the test gentle on the part. Vishay Dale’s selection guide says the current through a thermistor must be small enough to produce negligible self-heating error. The guide gives a worked example: a part with a 3 mW per degree C dissipation constant measured to 0.1 degree C should see no more than 3 × 0.1 × 0.5 = 0.15 mW. The pages opened do not state the test current of any specific multimeter, so check your manual if you need it. Our guide to resistance meters covers the basics.
The warm-and-cool routine
- Room reading. Let the thermistor sit in room air until it stops changing. Note the temperature of the room as well as the resistance.
- Warm it. Hold the thermistor body in your closed hand. The Notre Dame lab handout checks its own probe this way: about 10 kilohms at room temperature, decreasing when the probe is warmed in the hand.
- Cool it. Use an ice-water bath, as described below, or at least a cooler spot than the room.
- Return. Take the part back to room air. The Fred’s Appliance Academy page says the resistance should start to decrease immediately after removal from ice water and eventually match the room value again.
The size of the change gives you something to compare against. Using the Vishay NTCLE100E3 datasheet for a 10,000 ohm part, the 20 degree C value is 12,488 ohms and the 30 degree C value is 8,059 ohms. Our arithmetic: (8,059 − 12,488) ÷ 12,488 = −0.355, or a 35.5 percent fall for a 10 degree rise. The same sheet gives −4.38 percent per degree at 25 degrees C. Those figures belong to one part, so use your own part’s chart, but they show the scale: a few degrees of warming should move an NTC reading by several percent, which is easy to see on a meter.
An ice-water check for sealed probes
Several lab and training pages use an ice bath as a reference. The Walla Walla University lab says to add ice and water to a cup, keeping water to the minimum needed to fill the gaps between ice chunks, let it sit for a few minutes, immerse the thermistor, and stir while waiting for the resistance to settle. The Notre Dame handout asks for much more solid ice than liquid water. The Fred’s Appliance Academy page describes a refrigerator thermistor that read about 13,600 ohms before it went into the water, rose once submerged and reached 32,330 ohms in ice water, and says the service rule for that part is to replace it if the reading is open, shorted, or out of range by more than 10 percent of the manufacturer’s chart. Our arithmetic from those two readings is 32,330 ÷ 13,600 = 2.38, so the resistance rose to a little under two and a half times its starting value.
Two cautions apply. First, check how your part is built. The Vishay NTCLE100E3 datasheet says the coating on that radial part has no specified insulation properties, so do not assume a bare coated bead can sit in water. Use a sealed probe, or keep a bare part dry. Second, the lab pages use boiling water for the hot reference and one notes that liquid nitrogen is too cold and will break the thermistor. This guide does not recommend boiling water at home because of the scald risk, which is our own judgment. Iowa notes that hot tap water also works as the hot reference.
Reading the results
| What you see | What it may mean | Basis |
|---|---|---|
| OL at room temperature | Open element or broken lead, or a range that is too low | Fluke 87 manual, overload indication |
| Near zero ohms | Shorted part, or a path in parallel | Fluke guide, parallel paths |
| Reading never changes as you warm and cool | Dead NTC part, or a switching PTC part | Iowa demonstration; Vishay PTC note |
| Reading rises when warmed | Probably a PTC part, not an NTC part | TDK and Vishay descriptions |
| Reading drifts while you hold the probes | Finger contact, or body heat on the part | Fluke guide, body as a parallel path |
| Far from the chart value at a known temperature | Part out of tolerance, or the temperature is not what you think | Fred’s 10 percent service rule; Vishay tolerance |
Why a known temperature matters
A single room reading is only as good as your knowledge of the room. The Vishay NTCLE100E3 sheet puts the 10,000 ohm part’s slope at −4.38 percent per degree at 25 degrees C, so a 1 degree error in the temperature you assume moves the expected resistance by about 4.4 percent. The Vishay application note defines the resistance at 25 degrees C, substantially room temperature, as a convenient reference point for thermistors, and says tolerances on that value come from ceramic manufacture and chip dimensions. The Walla Walla lab adds that calibration is most accurate when the reference temperature is near the temperatures where the thermistor will be used. Use a thermometer you trust to record the room temperature; the pages opened do not rate the accuracy of any thermometer.
Mistakes to avoid
- Checking with power connected. The Fluke 87 manual says an external voltage makes an accurate resistance measurement impossible.
- Judging by one reading. One value cannot show direction or sensitivity; at least two temperatures can.
- Reading too soon. The labs wait for the resistance to settle, and the Walla Walla page stirs the bath while waiting.
- Treating a PTC part like an NTC part. A quiet hand-warm reading proves little for a switching PTC.
- Heating a bare part with a flame or boiling water. The sources use controlled labs, and a home setup has no such control.
How we chose
The routine comes from pages opened on 2026-10-21: a Fluke guide and the Fluke 87 manual for the meter side; Vishay documents for NTC and PTC behavior; a Vishay Dale selection guide for self-heating; and lab or training pages from Notre Dame, the University of Iowa, Walla Walla University and Fred’s Appliance Academy for the temperature routine. A step stays in only when one of those pages describes it, and our own arithmetic is labeled. We left out boiling-water work, in-circuit checks, and any appliance repair on mains-powered equipment.
What we could not verify
The pages opened do not give a pass or fail percentage that applies to every thermistor; the 10 percent figure belongs to one refrigerator service procedure. None of them states the test current of a given multimeter, a safe immersion rule for bare coated beads, or how long a hand-warm check should take. The chart values quoted are for one Vishay 10,000 ohm series, and your part will have its own chart. The sources also give no routine for checking a switching PTC part with a multimeter beyond its cold reading.
Frequently asked questions
What should a thermistor read at room temperature?
It should match its datasheet value at the temperature you have. Many thermistors are specified at 25 degrees C. The Notre Dame handout expects about 10 kilohms for its probe, but yours may differ.
How can I tell NTC from PTC with a multimeter?
Warm and cool the part and watch the direction. An NTC part falls when warmed. A ceramic PTC part stays low until it nears its switching temperature, then rises sharply, so hand warmth is unlikely to show much.
Why does the reading keep changing?
The part is still settling to the temperature around it, or your fingers are on the leads. Let it rest and keep your hands off the metal tips.
Can the meter damage a thermistor?
The pages opened do not state any meter’s test current. Vishay Dale advises keeping the current through a thermistor low enough to avoid self-heating error, so check your meter’s manual for its ohms-range test current.
Is a thermistor the same as a thermocouple?
No. A thermistor changes its resistance with temperature, which is why a resistance reading can check it. A thermocouple is a different sensor, covered in the guide linked above.
Bottom line
Take a room-temperature reading, move the temperature, and trust the direction and size of the change against the part’s own chart.
Sources
- Fluke, How to Measure Resistance with a Digital Multimeter: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-21)
- Fluke, 87 True RMS Multimeter Users Manual: https://assets.fluke.com/manuals/87______umeng0800.pdf (opened 2026-10-21)
- Vishay BCcomponents, NTC Thermistors application note (doc 29053): https://www.vishay.com/docs/29053/ntcappnote.pdf (opened 2026-10-21)
- Vishay BCcomponents, NTCLE100E3 datasheet (doc 29049): https://www.vishay.com/docs/29049/ntcle100.pdf (opened 2026-10-21)
- Vishay Dale, Selecting NTC Thermistors: https://www.vishay.com/docs/33001/seltherm.pdf (opened 2026-10-21)
- University of Notre Dame, AME 21216 Experiment A3 Sensor Calibration: https://prumbach.nd.edu/ame20216/a3/S26_A3_procedure.pdf (opened 2026-10-21)
- University of Iowa Physics and Astronomy, 5D20.50 Thermistors demonstration: https://instructional-resources.physics.uiowa.edu/node/5681 (opened 2026-10-21)
- Walla Walla University, ENGR 325 Lab 7 Measuring temperature: https://gab.wallawalla.edu/~larry.aamodt/engr384/lab7_21.pdf (opened 2026-10-21)
- Fred’s Appliance Academy, Testing a Sub-Zero Refrigerator Thermistor using a Glass of Ice Water: https://academy.fredsappliance.com/video/testing-a-sub-zero-refrigerator-thermistor-using-a-glass-of-ice-water/ (opened 2026-10-21)
- Vishay BCcomponents, PTCSL20 PTC thermistor datasheet (doc 29012), copy hosted by Farnell: https://www.farnell.com/datasheets/2245099.pdf (opened 2026-10-21)
- TDK EPCOS, PTC Thermistors product profile 2016: https://www.tdk-electronics.tdk.com/download/141178/22abc1c70cb555978705dd4188584463/ptc-thermistors-pp.pdf (opened 2026-10-21)
- OSHA, Working Safely with Electricity fact sheet: https://www.osha.gov/sites/default/files/publications/OSHA3942.pdf (opened 2026-10-21)
- OSHA, 29 CFR 1910.333 selection and use of work practices: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.333 (opened 2026-10-21)
- Fluke, IEC Category Ratings: Use the Right Tools for the Job: https://www.fluke.com/en-us/learn/blog/safety/iec-category-ratings-use-the-right-tools-for-the-job (opened 2026-10-21)
As an Amazon Associate we earn from qualifying purchases.
