Testing a temperature sensor requires a multimeter set to resistance mode and a way to change the sensor’s temperature in a controlled way. Most temperature sensors are resistive devices, meaning their electrical resistance changes predictably as they get hotter or colder. By measuring that resistance at a known temperature and comparing it to the manufacturer’s specifications, you can tell whether the sensor is working correctly.
What You Need
The only essential tool is a digital multimeter with a resistance setting, marked by the Ω (ohm) symbol on the dial. You’ll also want the sensor’s datasheet or resistance chart, which tells you what resistance to expect at specific temperatures. If you don’t have the original datasheet, searching the part number online will usually turn one up.
For a more thorough test, you’ll need a reliable heat source. A hair dryer, a cup of hot water, or an ice bath all work depending on the sensor type. The goal is to verify that resistance changes smoothly and in the right direction as temperature changes.
Basic Resistance Test at Room Temperature
Start by disconnecting the sensor from whatever circuit or system it’s part of. Set your multimeter to the resistance (ohms) setting. Touch the multimeter leads to the sensor’s two terminals and note the reading.
Compare this number to the expected resistance at room temperature from the sensor’s datasheet. For the most common type of sensor, a 10K NTC thermistor, you should see approximately 10,000 ohms at 25°C (77°F). If the reading shows “OL” (open line) or zero ohms, the sensor has failed.
How NTC and PTC Thermistors Behave
Temperature sensors come in two main varieties, and they respond to heat in opposite ways. Understanding which type you have determines what you should see on the multimeter when you apply heat.
NTC (negative temperature coefficient) thermistors are by far the most common in household appliances, HVAC systems, and automotive applications. Their resistance drops as temperature rises. A standard 10K NTC thermistor reads about 31,770 ohms at 0°C, 10,000 ohms at 25°C, and just 674 ohms at 100°C. That’s a dramatic change, which makes them easy to test.
PTC (positive temperature coefficient) thermistors work the opposite way. They show low resistance at room temperature, and that resistance climbs as they heat up. These are less common in everyday appliances but show up in motor protection circuits and some heating elements.
Testing With Heat
A room-temperature reading only tells you the sensor isn’t completely dead. To confirm it’s actually responding to temperature changes, you need to warm it up while watching the multimeter.
Keep the multimeter leads connected to the sensor’s terminals. Then apply gentle heat. For an NTC thermistor, you should see the resistance number falling steadily. For a PTC thermistor, it should rise. The change should be smooth and continuous. If the reading jumps erratically, doesn’t change at all, or suddenly drops to zero, the sensor is defective.
A hair dryer held a few inches away works well for sensors that are already removed from a system. For sensors mounted on a probe or designed for liquid contact, dipping them in warm water gives a more controlled result. Gradually increase the heat rather than blasting it to avoid damaging the sensor.
Testing a PT100 RTD Sensor
RTD sensors (resistance temperature detectors) are more precise than thermistors and are common in industrial and scientific equipment. The most widely used type is the PT100, which has a resistance of exactly 100 ohms at 0°C. Other variants include PT200, PT500, and PT1000, with resistances of 200, 500, and 1,000 ohms at 0°C respectively.
To test a PT100, set your multimeter to the lowest ohm range that covers a few hundred ohms. At room temperature (about 25°C), a working PT100 should read around 109 to 110 ohms. The resistance change per degree is much smaller than a thermistor, so you’ll need to look carefully at the reading. If the sensor reads significantly outside the expected range, or shows open or shorted, it needs replacing.
Calibration With an Ice Bath
An ice bath gives you a reliable 0°C (32°F) reference point that costs nothing to make. This is the simplest way to check whether a temperature sensor is reading accurately rather than just confirming it responds to temperature changes.
Fill a container all the way to the top with ice. Crushed ice works best because it packs together with fewer air gaps, but cubed ice is fine. Slowly add water to fill the spaces between the ice, stopping about half an inch below the top of the ice. Let the mixture sit for a minute or two. The key detail: if the ice starts floating off the bottom, pour off some water and add more ice. Water sitting below the ice layer won’t be at 32°F.
Submerge the sensor in the ice bath and measure its resistance. Compare that reading to the datasheet value at 0°C. For a 10K NTC thermistor, you should see roughly 31,770 ohms. For a PT100 RTD, you should see 100 ohms.
Calibration With Boiling Water
Boiling water provides a second reference point, but only if you account for your altitude. At sea level, water boils at 212°F (100°C). For every 500 feet of elevation above sea level, the boiling point drops by about 0.9°F. At 5,000 feet, water boils at 203°F. At 8,000 feet, it’s down to 197.4°F.
This matters because if you’re testing a sensor in Denver (about 5,280 feet), your boiling water isn’t actually at 100°C. You’ll need to look up the expected resistance for the actual boiling point at your altitude, not the textbook 100°C value. A sensor that reads “wrong” in a boiling water test at elevation might be perfectly accurate.
Testing HVAC Temperature Sensors
HVAC systems use NTC thermistors throughout, from the outdoor ambient sensor to the supply air and refrigerant line sensors. If your system is throwing temperature-related error codes, testing these sensors is straightforward.
Disconnect the sensor from the control board and measure its resistance. Then compare your reading to the manufacturer’s resistance chart. For a typical HVAC outdoor sensor at 70°F, expect about 11,942 ohms. At 40°F, that climbs to roughly 26,092 ohms. At 0°F, it jumps to around 86,463 ohms. If the reading is below about 1,000 ohms, the sensor is shorted. If it’s above 350,000 ohms, it’s effectively open and has failed.
Supply air (bonnet) sensors operate in a warmer range. At 110°F, expect about 4,663 ohms. At 130°F, about 3,047 ohms. A reading below 200 ohms or above 100,000 ohms on these sensors indicates failure.
One useful shortcut: measure the sensor’s resistance, then check the outdoor temperature with a separate thermometer. If the resistance you measured matches the chart value for that temperature, the sensor is good. If it’s way off, replace it.
Signs of a Failed Sensor
A completely dead sensor will show one of two extremes on your multimeter. An “OL” or infinite resistance reading means the sensor’s internal element has broken, creating an open circuit. A reading of zero or near-zero ohms means the sensor has shorted internally. Either condition means the sensor needs to be replaced.
Partial failures are trickier. A sensor might read correctly at room temperature but become erratic or inaccurate at higher temperatures. This is why testing at multiple temperature points, such as an ice bath and warm water, gives you much more confidence than a single room-temperature check. If the resistance changes smoothly and matches the datasheet values at two or more known temperatures, the sensor is working properly.