General Assist Technical Reference
Thermistor Testing
How to test temperature thermistors and PCB thermistor supply voltage
Use this procedure when diagnosing suspected thermistor faults, incorrect temperature readings or thermistor-related error codes.
Key principle: A temperature sensing fault can be caused by either the thermistor itself or the PCB supplying/reading the thermistor circuit. Test both before replacing parts.
Before testing: Record the actual ambient/component temperature first. Resistance and signal voltage must be compared against temperature, so an accurate temperature reference is required.
1. Testing thermistor resistance
- Isolate power to the unit before unplugging the thermistor from the PCB.
- Disconnect the thermistor plug from the PCB.
- Set the multimeter to resistance (Ω).
- Access the metal contacts on the rear of the thermistor connector and measure across the two thermistor conductors.
- If the meter probes are too large to contact the terminals, use a suitable fine test lead, short length of wire or similar conductive adaptor to reach the metal contact without damaging the connector.
- Compare the measured resistance with the appropriate temperature/resistance table for the thermistor being tested.
Expected behaviour: These thermistors are temperature dependent. The resistance must correspond reasonably with the actual temperature at the sensor. An open circuit, short circuit or value substantially outside the expected curve indicates a thermistor or wiring fault.
2. Testing PCB voltage to the thermistor
The thermistor supply/read circuit can be checked from either the top side or bottom side of the PCB with the thermistor connected.
Important: When checking the thermistor circuit for DC voltage, use a suitable DC neutral/reference. The source procedure identifies the black wire in the fan motor plug as the DC neutral/reference point.
| Check | Expected result |
|---|---|
| Thermistor connector – supply side | One pin should have approximately 5 V DC. |
| Thermistor connector – signal side | The other pin will have a DC voltage that varies with thermistor temperature. |
| Incorrect supply/signal voltage | Indicates a likely PCB/circuit problem once connector, wiring and reference point have been confirmed. |
3. Thermistor reference readings
The values below are the reference readings supplied with the thermistor testing guide. Use the table that matches the thermistor type being tested.
Room temperature thermistor
| °C | Resistance (kΩ) | Voltage (V) | °C | Resistance (kΩ) | Voltage (V) |
|---|---|---|---|---|---|
| 0 | 33.62 | 1.15 | 50 | 3.69 | 3.66 |
| 5 | 25.93 | 1.39 | 60 | 2.98 | 3.85 |
| 10 | 20.18 | 1.66 | 70 | 2.47 | 4.00 |
| 15 | 15.84 | 1.94 | 80 | 2.09 | 4.14 |
| 20 | 12.54 | 2.22 | 90 | 1.93 | 4.37 |
| 25 | 10.00 | 2.50 | 100 | 0.81 | 4.63 |
| 30 | 8.01 | 2.77 | — | — | — |
| 35 | 6.51 | 3.03 | — | — | — |
| 40 | 5.30 | 3.27 | — | — | — |
| 45 | 4.35 | 3.48 | — | — | — |
Indoor heat exchanger thermistor
| °C | Resistance (kΩ) | Voltage (V) |
|---|---|---|
| 0 | 176.03 | 1.10 |
| 5 | 134.23 | 1.36 |
| 10 | 103.34 | 1.63 |
| 15 | 80.26 | 1.92 |
| 20 | 62.91 | 2.21 |
| 25 | 49.70 | 2.51 |
| 30 | 39.67 | 2.79 |
| 35 | 31.74 | 3.06 |
| 40 | 25.64 | 3.30 |
| 50 | 17.06 | 3.73 |
Outdoor heat exchanger / outdoor temperature thermistors
These sensors use different resistance curves. Always identify the sensor type before comparing readings. Representative values from the supplied reference chart are shown below.
| Temperature | Outdoor heat exchanger | Outdoor temperature |
|---|---|---|
| -20°C | 88.43 kΩ / 0.589 V | 133.30 kΩ / 1.12 V |
| 0°C | 20.54 kΩ / 0.928 V | 42.80 kΩ / 2.36 V |
| 20°C | 6.09 kΩ / 2.01 V | 12.10 kΩ / 3.77 V |
| 40°C | 2.59 kΩ / 3.20 V | 4.31 kΩ / 4.47 V |
| 60°C | 1.21 kΩ / 4.12 V | 2.07 kΩ / 4.74 V |
Discharge / compressor temperature thermistor
The discharge and compressor temperature tables in the source use the same high-temperature curve. Representative values are:
| °C | Resistance (kΩ) | Voltage (V) |
|---|---|---|
| -30 | 931.50 | 0.07 |
| 0 | 168.60 | 0.36 |
| 20 | 63.88 | 0.85 |
| 40 | 27.09 | 1.62 |
| 60 | 12.51 | 2.31 |
| 80 | 6.46 | 3.13 |
| 100 | 3.98 | 3.83 |
| 120 | 2.24 | 4.26 |
Reference note: Different product families can use different thermistor curves. Always use the applicable service manual or model-specific thermistor table where one is available.
4. Diagnosis decision
| Test result | Action |
|---|---|
| Resistance does not match temperature / open / short | Check thermistor wiring and connector. If wiring is sound, replace the thermistor. |
| Resistance is correct but PCB voltage is incorrect | Check connector, PCB tracks and DC reference. If these are sound, the PCB is suspect. |
| Resistance and voltage both appear correct | Check for intermittent connector/wiring issues and confirm the thermistor is correctly mounted in the required position. |
Quick rule: If the resistance test fails, change the thermistor. If the DC voltage test fails after wiring and reference checks, investigate/replace the PCB.