Piotr Wytrazek/iStock/GettyImages

Things Needed

• Soldering iron
• Multimeter or ohmmeter

Thermistors are temperature‑sensitive resistors that adjust their resistance in response to temperature changes. They are integral to many electronic circuits, and a faulty thermistor can compromise the entire system. Testing a thermistor involves applying controlled heat while monitoring its resistance.

Step 1: Set the Multimeter to Resistance Mode

Configure your multimeter for ohm measurement. This mode allows you to read the thermistor’s resistance accurately.

Step 2: Connect the Thermistor to the Multimeter

Attach the multimeter leads to the thermistor terminals. Polarity is irrelevant for this test, so the order of the leads does not matter.

Step 3: Apply Heat with a Soldering Iron

Heat the soldering iron until its tip is warm, then gently bring the tip close to the thermistor. Avoid direct contact to prevent damage.

Step 4: Observe the Resistance Reading

As the thermistor warms, watch the multimeter. A properly functioning positive temperature coefficient (PTC) thermistor will show a steady, smooth increase in resistance. A negative temperature coefficient (NTC) thermistor will exhibit a smooth, steady decrease.

Step 5: Identify Faulty Behavior

Indicators of a bad thermistor include a flat reading that does not change, a sudden jump to zero or infinite resistance, or erratic fluctuations. Any of these signs mean the component should be replaced.