Purpose
To verify the accuracy and operation of thermistors installed in Fujitsu outdoor units by checking their resistance values against standard reference tables.
This document supports fault diagnosis, sensor verification, and board-level testing in both field and workshop service conditions.
1. Overview of Outdoor Thermistors
Outdoor units typically use multiple thermistors for system monitoring and control:
| Thermistor Type | Location / Function | Designation |
|---|---|---|
| Discharge Thermistor | Measures discharge pipe temperature to protect compressor from overheating | TH1 |
| Compressor Thermistor | Monitors compressor body temperature | TH2 |
| Outdoor Heat Exchanger Thermistor | Controls defrost and cooling cycle performance | TH3 |
| Outdoor Air Temperature Thermistor | Provides ambient reference for operation control | TH4 |
| Heat Sink Thermistor (if fitted) | Monitors inverter module temperature | TH5 |
2. Checking Thermistor Resistance
Procedure:
Isolate Power: Ensure power is disconnected from the outdoor unit.
Remove Connector: Disconnect thermistor from the control PCB.
Measure Resistance: Using a digital multimeter, set to the kΩ range, measure across thermistor terminals.
Compare Reading: Refer to the reference tables below.
Judgement: Replace thermistor if resistance deviates by more than ±10% from specified values.
Testing Equipment Example:
Multimeter: Set to resistance (Ω) mode.
Ambient reference thermometer: To confirm surrounding temperature.
3. Reference Resistance Values
Outdoor Thermistor Standard Reference (°C / kΩ)
| Temperature (°C) | Discharge Temp TH (A) | Heat Exchanger TH (B) | Outdoor Temp TH (C) | Heat Sink TH (D) |
|---|---|---|---|---|
| -20 | — | — | 105.4 | — |
| -10 | — | 27.8 | 58.2 | 27.4 |
| -5 | — | 21.0 | 44.0 | 20.7 |
| 0 | 168.6 | 16.1 | 33.6 | 15.8 |
| 5 | 129.8 | 12.4 | 25.9 | 12.2 |
| 10 | 100.9 | 9.6 | 20.2 | 9.5 |
| 15 | 79.1 | 7.6 | 15.8 | 7.5 |
| 20 | 62.6 | 6.0 | 12.5 | 5.9 |
| 25 | 49.8 | 4.8 | 10.0 | 4.7 |
| 30 | 40.0 | 3.8 | 8.0 | 3.8 |
| 40 | 26.3 | 2.5 | 5.3 | 2.5 |
| 50 | 17.8 | 1.7 | 3.6 | 1.7 |
| 60 | 12.3 | 1.2 | 2.5 | 1.2 |
| 70 | 8.7 | — | — | 0.8 |
| 80 | 6.3 | — | — | 0.6 |
| 90 | 4.6 | — | — | 0.4 |
| 100 | 3.4 | — | — | 0.3 |
| 110 | 2.6 | — | — | — |
| 120 | 2.0 | — | — | — |
4. Typical Resistance Characteristics
Each thermistor follows an NTC (Negative Temperature Coefficient) curve, meaning resistance decreases as temperature increases.
Examples from Outdoor Systems:
| Sensor | Typical Resistance @25°C (kΩ) | Function Curve Trend |
|---|---|---|
| Discharge TH | ~50 kΩ | Steep decline from 150 kΩ @0°C to 3 kΩ @100°C |
| Compressor TH | ~50 kΩ | Similar to discharge TH, lag slightly slower response |
| Heat Exchanger TH | ~10 kΩ | Used for defrost initiation |
| Outdoor Temp TH | ~10 kΩ | Used for ambient reference |
| Heat Sink TH | ~5 kΩ | Faster drop due to heat conduction from IPM |
Graphical representation (from attached diagrams):
Exponential decay curve with resistance rapidly decreasing from ~100 kΩ at 0°C to <10 kΩ above 60°C.
Voltage output corresponds inversely to resistance via the divider circuit on the PCB (e.g., 5 V input → 0.3–4.5 V output range).
5. Voltage Correspondence (For PCB Measurement)
Typical voltage across thermistor circuit (relative to GND) during operation:
| Temperature (°C) | Expected Voltage (V) |
|---|---|
| -10°C | 4.5 – 4.8 V |
| 0°C | 4.0 – 4.3 V |
| 25°C | 3.0 – 3.3 V |
| 50°C | 2.0 – 2.5 V |
| 80°C | 1.0 – 1.5 V |
(Values may vary slightly depending on model and resistor network tolerance.)
6. Fault Diagnosis
| Symptom | Probable Cause | Action |
|---|---|---|
| Unit does not start in heating or cooling | Open circuit thermistor | Check resistance — infinite reading indicates fault |
| Unit stops with error code (e.g. Er40, Er41, Er53) | Shorted or out-of-range thermistor | Replace sensor |
| Defrost cycles abnormally | Incorrect outdoor heat exchanger TH | Compare resistance vs actual coil temperature |
| Compressor stops unexpectedly | Discharge or compressor thermistor over-reading | Confirm thermistor mounting and contact with pipe |
7. Key Service Notes
Always measure at the PCB connector for circuit verification before removing thermistor.
If multiple sensors read incorrectly, check 5 V DC reference on control board.
Use OEM thermistors only – differing B-constants affect control accuracy.
Replace thermistor if physical damage, corrosion, or water ingress is present.
8. Summary
Thermistors are critical in Fujitsu inverter systems for temperature-based control and protection.
Expected resistance range is ~50 kΩ at 25°C, decreasing exponentially with temperature.
Verification involves resistance and voltage cross-check against standard charts.
Accurate diagnosis prevents unnecessary PCB or compressor replacement.
