AIRSTAGE Technical Reference
Thermistor Testing and Diagnostics
Voltage, resistance and temperature correlation checks
For service technicians diagnosing temperature-sensor circuits in Fujitsu air conditioning systems
Thermistors monitor room air, refrigerant circuits and electronic modules. Correct sensor feedback supports compressor control, defrost operation and fan modulation.
Electrical safety: Live voltage testing must only be performed by suitably qualified personnel. Isolate power completely before disconnecting a thermistor or carrying out resistance checks.
1. Understanding thermistor operation
- Thermistors are NTC (Negative Temperature Coefficient) devices, meaning resistance decreases as temperature increases.
- The control PCB supplies a 5 V DC reference through a resistor network.
- The sensor divides this voltage and returns a temperature-dependent feedback signal to the PCB.
| Temperature | Resistance | Voltage |
|---|---|---|
| 0°C | 176 kΩ | 1.1 V DC |
| 20°C | 63 kΩ | 2.2 V DC |
| 30°C | 40 kΩ | 2.8 V DC |
2. Power-on test: live voltage check
Purpose: Confirm that the thermistor circuit receives the correct PCB supply voltage and returns a stable, temperature-dependent signal.
- Power on the system and allow operation to stabilise for several minutes.
- Set the multimeter to DC voltage.
- Measure between ground (0 V) and the thermistor supply pin. The expected reading is approximately 5.0 V DC.
- Measure between ground (0 V) and the thermistor signal pin. The return should be below 5 V DC and vary with temperature.
- Convert the measured voltage using the relevant thermistor chart or the M Technician App.
Example: An outdoor temperature thermistor reading of 3.83 V and 11.6 kΩ corresponds to approximately 21.8°C. This is consistent with the source reference of approximately 12.6 kΩ and 3.8 V at 20°C.
Stable ground point: Use a reliable 0 V reference, such as the black wire on the pressure transducer or the relevant control PCB ground point.
3. Power-off test: resistance check
Purpose: Confirm thermistor integrity and temperature correlation independently of the control PCB.
- Isolate power to the system completely.
- Disconnect the thermistor from the PCB.
- Set the multimeter to resistance in kΩ.
- Measure directly across the thermistor leads.
- Compare the reading with the applicable temperature/resistance chart.
| Thermistor | Temperature | Resistance | Typical DC voltage |
|---|---|---|---|
| Room temperature | 20°C | 12.53 kΩ | 2.21 V |
| Indoor HEX | 20°C | 62.90 kΩ | 2.21 V |
| Discharge temperature | 20°C | 64.50 kΩ | 0.83 V |
| Compressor temperature | 20°C | 62.55 kΩ | 0.86 V |
| Outdoor HEX | 20°C | 6.10 kΩ | 2.18 V |
| Outdoor temperature | 20°C | 12.64 kΩ | 3.75 V |
| Heat sink | 20°C | 6.10 kΩ | 2.18 V |
4. Diagnostic interpretation
| Reading | Possible cause | Service action |
|---|---|---|
| 0 V / 0 Ω | Short-circuit thermistor | Replace thermistor |
| 5 V / infinite resistance | Open-circuit thermistor | Replace thermistor |
| Unstable voltage | Poor connection or moisture ingress | Reseat the connection or replace the sensor |
| Voltage appears correct but fault remains | Possible PCB input fault | Verify wiring continuity and assess PCB replacement if required |
5. Technician tips
- Allow the system to stabilise before testing. Rapid temperature changes can produce misleading readings.
- Compare related sensors, such as outdoor HEX and outdoor ambient, for logical consistency.
- Do not assume all sensor types use the same resistance curve. Use the reference data for the relevant model and thermistor type.
- Use the M Technician App or AIRSTAGE service tool where available to convert voltage and resistance to temperature.
- When results are uncertain, compare against a known-good unit or the official thermistor chart for the model series.
Key checks: Confirm the 5 V DC supply, verify a stable return voltage, and compare measured resistance against the actual sensor temperature.
Technical values may vary by model and thermistor type. Always confirm readings against the applicable service documentation before replacing components.