A thermistor is a resistor whose value changes with temperature. Laptops use them to watch the battery, the charger, the voltage regulators and other hot spots, and to decide when to speed up fans, slow down the processor, stop charging or shut down. When a thermistor fails, the laptop does not see a broken part — it sees an impossible temperature, and it reacts accordingly. This guide explains how to test thermistors and how to recognise the symptoms they cause.
NTC and PTC
| Type | How resistance changes | Typical use on a laptop |
|---|---|---|
| NTC (negative temperature coefficient) | Falls as temperature rises | Temperature sensing: battery, charger, regulators, skin temperature |
| PTC (positive temperature coefficient) | Rises as temperature rises, often sharply above a threshold | Over-temperature detection, resettable fuses |
On laptop boards, NTC thermistors used for sensing are the most common. They are rated by their resistance at 25 °C — often written R25 — and by a constant (often called B) that describes how fast the resistance changes with temperature. A common value is 10 kΩ at 25 °C, but boards use other values too. The glossary entry for ntc thermistor explains the curve.
Where they are
- Inside the battery pack, connected to the laptop through a dedicated pin.
- Near the charger IC and its MOSFETs, to protect the charging circuit.
- Near processor and graphics voltage regulators, to reduce power if the regulator overheats.
- On the palm rest or bottom area, for skin temperature limits.
- Near SSDs or other hot components, on some designs.
In the schematic, thermistors may use the designator RT, RH, TH or simply R, with a resistor symbol crossed by a diagonal line. In the boardview, they look like ordinary resistors or capacitors, usually in small packages.
Testing out of circuit
- Set the meter to resistance.
- Measure across the thermistor at room temperature.
- Compare with the rated value — R25 from the schematic or the part's marking.
Room temperature is rarely exactly 25 °C. A room slightly colder than 25 °C makes an NTC read slightly higher than R25; a warmer room, slightly lower. Small differences are normal.
The warm-up test
The quickest way to confirm a thermistor responds:
- Keep the probes on the thermistor.
- Warm it gently with your fingertip or a brief, low-temperature puff of hot air from a distance.
- Watch the reading.
An NTC's resistance should fall noticeably and then recover as it cools. A PTC's resistance should rise. A reading that does not change at all suggests a damaged part.
Testing in circuit
A sensing thermistor is usually part of a voltage divider: a fixed resistor from a reference voltage, and the thermistor to ground (or the other way round). The chip that reads the temperature — the EC, the charger IC or a regulator controller — measures the voltage at the point between them.
Two in-circuit checks:
- Unpowered, resistance across the thermistor — the reading includes the fixed resistor and other parts in parallel, so it is lower than the thermistor's own value. Compare with an identical position on a known-good board, or lift one end for a clean reading. Testing resistors in circuit follows the same logic, described in How to Test a Resistor with a Multimeter (In and Out of Circuit).
- Powered, voltage at the sense node — measure the voltage between the thermistor and the fixed resistor. Calculate the expected voltage from the divider and the thermistor's value at room temperature, and compare. A reading at the reference voltage or at zero means the thermistor or the fixed resistor is open or shorted.
How the laptop reacts to a bad thermistor
What the laptop sees depends on how the divider is wired. For an NTC connected to ground with a pull-up resistor:
| Thermistor fault | Sense voltage | What the chip reads |
|---|---|---|
| Open | Pulled to the reference | Extremely cold |
| Shorted | Pulled to zero | Extremely hot |
| Drifted high | Higher than normal | Colder than reality |
| Drifted low | Lower than normal | Hotter than reality |
If the thermistor is connected the other way round, the readings are reversed. Check the schematic for your board.
Typical symptoms:
- Battery does not charge, or charges only at a reduced rate — the battery or charger thermistor reads out of range. More on charging faults in Laptop Not Charging: Charging Circuit Diagnosis.
- Fans run at full speed constantly — the EC reads an excessive temperature, or treats a missing sensor as a fault.
- Processor throttles heavily — a regulator thermistor reads too hot.
- Laptop shuts down unexpectedly — a reading crosses a shutdown threshold. See thermal shutdown.
The battery thermistor
The battery's thermistor is inside the pack, but the laptop reads it through the battery connector. With the battery removed, measure between the thermistor pin and pack negative. A reading in kilohms that responds to warmth means the thermistor is fine; OL means the thermistor or its connection inside the pack is broken. On the laptop side, check continuity from the connector's thermistor pin to the sensing chip, and the pull-up voltage on a powered board. The connector pinout is explained in The Laptop Battery Connector and SMBus Explained.
How thermistors fail
- Open — a cracked body after impact or board flexing, or a corroded joint after liquid damage.
- Drifted — overheating can change the value permanently.
- Shorted — rare in the thermistor itself; more often a solder bridge or corrosion across its pads.
Replacing a thermistor
Match the R25 value, the B constant, the type (NTC or PTC) and the package. A thermistor with the right R25 but a different B constant reads correctly at room temperature but gives wrong readings at other temperatures — exactly where accuracy matters most.
Example: a laptop that refuses to charge after a drop
After a drop, a laptop runs from its charger but will not charge the battery, though the battery works in another identical laptop. The charger IC's datasheet shows a thermistor input used to monitor temperature near the charger. On the powered board, the voltage at that input sits at the reference voltage — the chip reads the area as extremely cold and blocks charging. Unpowered, the thermistor reads OL; under the microscope, its body has a hairline crack. A new thermistor with the same ratings restores charging.
FAQ
What value should my thermistor read?
Its rated value at 25 °C, adjusted for the actual room temperature. The schematic or the part number gives the rated value.
Can I replace a thermistor with a fixed resistor?
It may make the laptop work at room temperature, but the temperature protection is lost. It is not a safe repair, especially for battery and charger sensing.
Why does my thermistor read lower in circuit?
Other parts in the divider and on the same nets are in parallel with it. Lift one end for an exact value.
How do I tell an NTC from a PTC?
The schematic or part number tells you. In practice, warm the part while measuring: an NTC's resistance falls, a PTC's rises.
Can a thermistor cause the fans to run at full speed?
Yes. If the EC reads an impossible temperature or a missing sensor, many designs run the fans at full speed as a safety measure.
Bottom line
Measure the thermistor at room temperature, warm it and watch the reading change, and compare with its rated value. In circuit, measure the voltage at the sense node and compare with the divider calculation. A bad thermistor makes the laptop believe something is too hot or too cold, which shows up as charging, fan or throttling problems rather than as an obvious failure.