Thermistor
A temperature sensor whose electrical resistance changes predictably with temperature. Sensor fault codes normally mean the board measured a resistance that is open, shorted, or out of range, which can be the sensor, the plug, or the wiring to it.
Also called temperature sensor, NTC sensor
What is a thermistor?
A thermistor is a temperature sensor whose resistance changes with temperature. The control board reads that resistance and converts it to a temperature using a table published for the part.
Thermistor: how it works
The board applies a small voltage across the sensor and measures what comes back, then looks the resulting resistance up against a table for that part. A value outside the table at either end means open circuit or short circuit, and because the whole measurement travels through a connector and a length of wire, any break in that path produces the same reading as a failed sensor.
Where you meet the term
- In a temperature sensor fault code, where the board saw a resistance outside its expected range.
- In a coil temperature protection code, where the sensor is working correctly and reporting a genuine condition.
- In a service manual appendix, where the resistance table for each sensor is published.
How to check it
- Measure resistance at the plug and compare with the table. A reading taken at a known temperature and checked against the published table separates a drifted sensor from a broken connection. Technician
- Inspect the connector and the harness route. Resistance falls as temperature rises on the negative-coefficient type used here, so the curve runs the opposite way to intuition. Technician
Thermistor: what it is not
- A thermistor that reads a plausible but wrong value raises no code at all. Only a reading outside the expected range is caught.
- A sensor reporting an alarming temperature may be entirely healthy. Confirm the reading is wrong before replacing the part that reported it.
Thermistor questions
What is an NTC thermistor?
Negative temperature coefficient: resistance falls as temperature rises. It is the common type in HVAC sensing, and the manufacturer publishes a resistance value for each temperature.
Can a thermistor drift rather than fail outright?
Yes, and that is harder to find, because the board sees a plausible value and acts on it. A reading compared against an independent thermometer at the same point is what exposes it.
Why do sensor faults appear after a repair?
Connectors disturbed during other work are a recognised cause. A pin not fully seated makes intermittent contact and produces a sensor code that comes and goes.
Where this definition comes from
Written from service manual, manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- 15 and 19 Series Service ManualDaikin 15 and 19 Series units documented in service manual SiUS041501E.
- E1, EL 01, or EL01 Error CodesDiscontinued Quantum WYS, CYB, RYB, UYB, and FYB series plus listed companion models.