Outdoor Thermistor Testing
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.
9. Reference diagrams
Use the following diagrams as supporting references when comparing thermistor resistance and circuit voltage characteristics.
