General Assist Technical Reference
Thermistors - Quick Test Procedure
Resistance and DC supply checks
Field diagnosis of two-pin NTC thermistors used on indoor and outdoor HVAC equipment.
This procedure provides a repeatable method to identify two common thermistor faults: a failed sensor element with resistance outside specification, or an incorrect PCB-supplied DC voltage.
Safety: Isolate mains power before unplugging or handling thermistor connectors. Restore power only when instructed for live DC checks. Keep hands and tools clear of moving parts and use the DC voltage range for PCB supply checks.
1. Tools and preparation
- Digital multimeter capable of measuring resistance (Ω) and DC voltage.
- Fine probes or a short piece of thin conductor such as wire or a paperclip if standard probe tips are too large.
- Ambient thermometer to establish the reference temperature.
- Insulated gloves and eye protection.
- Thermistor Reading Chart for the applicable model family, showing resistance and voltage against ambient temperature.
2. Establish the ambient reference
- Measure and record the ambient temperature close to the thermistor location.
- Locate the corresponding resistance and voltage values in the applicable Thermistor Reading Chart.
3. Thermistor resistance test - power OFF
Purpose: Confirm the thermistor sensor element is operating correctly.
- Switch power OFF to the unit.
- Unplug the thermistor connector from the PCB.
- Set the multimeter to resistance (Ω).
- Turn the thermistor connector over to expose the metal strips. If the probe tips are too large, use a short piece of thin wire or a paperclip around each probe to make contact with the terminals.
- Measure resistance across the two thermistor pins.
- Compare the measured value with the Thermistor Reading Chart for the recorded ambient temperature.
| Result | Interpretation |
|---|---|
| Pass | Reading aligns with the Thermistor Reading Chart within the expected tolerance. |
| Fail | Reading is open-circuit, short-circuit, or significantly outside the expected value. Replace the thermistor. |
4. PCB DC voltage supply test - power ON
Live test: This step requires power to be restored. Exercise appropriate electrical safety controls while taking measurements.
Purpose: Confirm the PCB is supplying the correct DC voltage to the thermistor circuit.
- Reconnect the thermistor to the PCB.
- Restore power.
- Set the multimeter to DC voltage.
- Use a solid DC neutral reference. The black wire in the fan-motor plug is a suitable DC neutral.
- Probe the two thermistor circuit pins with the connector installed, either from the top of the board or from the underside of the PCB where access permits.
| Test point | Expected reading |
|---|---|
| Supply pin | Approximately +5 V DC. |
| Signal pin | Temperature-dependent DC voltage that corresponds with the ambient temperature and the Thermistor Reading Chart. |
Pass: Approximately +5 V DC is present on one pin and the second pin shows the expected temperature-correlated voltage.
Fail: The +5 V DC supply is missing or incorrect, or the temperature-related voltage is unreasonable. Confirm wiring and connectors before replacing the PCB.
Fail: The +5 V DC supply is missing or incorrect, or the temperature-related voltage is unreasonable. Confirm wiring and connectors before replacing the PCB.
5. Quick decision flow
| Finding | Action |
|---|---|
| Resistance outside the expected range with power OFF. | Replace thermistor. |
| Resistance is correct, but the live PCB test fails due to missing ~5 V DC or incorrect temperature voltage. | Confirm wiring/connectors, then replace PCB if the fault remains. |
6. Tips and common pitfalls
- Always verify ambient temperature first, as both resistance and DC voltage expectations depend on temperature.
- Poor probe contact at the thermistor connector can create misleading resistance readings. Use fine wire or a paperclip if required.
- Use the DC voltage range for supply checks. An AC range can produce misleading results and mask a PCB issue.
7. Frequently asked questions
Can I test resistance with the thermistor still connected to the PCB?
No. For a valid resistance measurement, disconnect the thermistor from the PCB first.
No. For a valid resistance measurement, disconnect the thermistor from the PCB first.
What if I cannot reach the pins from the top of the PCB?
The DC voltage test can be performed from the underside of the PCB with the thermistor connected, using the DC neutral reference.
The DC voltage test can be performed from the underside of the PCB with the thermistor connected, using the DC neutral reference.
What reference voltage should I expect?
Approximately +5 V DC on one pin. The second pin varies with temperature according to the Thermistor Reading Chart.
Approximately +5 V DC on one pin. The second pin varies with temperature according to the Thermistor Reading Chart.