Description
Thermistors are critical temperature sensors used to monitor operating conditions throughout Fujitsu systems, including room air, refrigerant circuits, and electronic modules.
Proper thermistor function ensures correct control of compressor operation, defrost cycles, and fan modulation.
This guide outlines standard voltage and resistance testing procedures for diagnosing thermistor operation in both powered and isolated (power-off) conditions.
1. Understanding Thermistor Operation
Thermistors are NTC (Negative Temperature Coefficient) devices — resistance decreases as temperature increases.
The control PCB supplies a 5V DC reference to each thermistor circuit through a resistor network.
The thermistor divides this voltage, providing a feedback signal voltage proportional to temperature.
Typical control logic:
| Temperature (°C) | Resistance (kΩ) | Voltage (V DC) |
|---|---|---|
| 0 | 176 | 1.1 |
| 20 | 63 | 2.2 |
| 30 | 40 | 2.8 |
2. Power-On Test (Live Voltage Test)
Purpose:
To confirm the thermistor circuit is receiving correct voltage from the PCB and providing an accurate temperature-dependent return signal.
Procedure:
Power on the system and allow operation for several minutes.
Set the multimeter to DC voltage mode.
Measure voltage:
Between ground (0V) and thermistor supply pin → should read approx. 5.0 V DC.
Between ground (0V) and thermistor signal pin → should read below 5 V DC, proportional to temperature.
Use the thermistor voltage chart or M Technician App to convert voltage to equivalent temperature.
Example Measurement:
Outdoor temperature thermistor reading:
Voltage: 3.83 V
Resistance: 11.6 kΩ
Approximate Temperature: 21.8°C
(Reading consistent with expected 12.6 kΩ @ 20°C ≈ 3.8 V)
Tip:
Use a reliable ground point such as the black wire on the pressure transducer or the control PCB’s earth terminal for stable readings.
3. Power-Off Test (Resistance Check)
Purpose:
To confirm thermistor integrity and temperature correlation independently of the control board.
Procedure:
Isolate power to the system completely.
Unplug the thermistor from the PCB.
Set the multimeter to resistance (kΩ) mode.
Measure directly across thermistor leads.
Compare with reference table values.
Example Reference Values:
| Thermistor | Temperature | Resistance | DC Voltage (typical) |
|---|---|---|---|
| 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 / ∞ Ω | Open circuit thermistor | Replace thermistor |
| Voltage unstable | Poor connection / moisture ingress | Reseat or replace sensor |
| Voltage correct but system fault remains | PCB input fault | Verify continuity and replace PCB if required |
5. Technician Tips
Allow the system to stabilize before testing; rapid temperature change can cause misleading readings.
Compare multiple sensors (e.g., outdoor HEX vs. outdoor ambient) for consistency.
Thermistor should read approximately 12 kΩ at 20°C across most sensor types.
The M Technician App or Airstage service tool provides on-site conversion between voltage, resistance, and temperature.
Best practice:
When uncertain, cross-check your measured values with known-good units or the official Fujitsu thermistor chart for that model series.
6. Summary
Confirm 5V DC supply and stable return voltage under power.
Verify resistance vs temperature correlation under power-off conditions.
Typical outdoor thermistor readings at ~20°C are 12.5–13 kΩ with ~3.8 V return.
Faulty thermistors cause erratic temperature control, defrost issues, or inverter protection errors.
Power Off Test:
Unplug the thermistor.
Measure resistance in kΩ and compare with reference values.
Tips: Use a reliable ground point, such as the black wire of a pressure transducer sensor.
