Thermistors — Quick Test Procedure (Resistance & DC Supply Check)

Summary

This article outlines a safe, repeatable method to test HVAC thermistors using a multimeter. It covers two fault modes—failed sensor element (resistance out of range) and incorrect PCB-supplied DC voltage—plus how to reference ambient temperature and interpret results.

Applies to

  • Field diagnosis of indoor/outdoor temperature thermistors on legacy and current platforms where a two-pin NTC thermistor connects to the PCB.

Tools & Preparation

  • Digital multimeter (Ω, V DC)

  • Fine probes or thin conductor (paperclip or wire) to contact clip terminals if probes are too large

  • Ambient thermometer (for reference temperature)

  • Insulated gloves and eye protection

  • Access to the Thermistor Reading Chart for the model family (resistance & voltage vs. ambient)

Safety

  • Isolate mains power before unplugging or handling thermistor connectors. Restore power only when instructed for live DC checks.

  • Keep hands/tools clear of moving parts.

  • Measure DC supply on DC range only.

Procedure

Step 1 — Establish Ambient Reference

  1. Measure and note the ambient temperature near the thermistor location.

  2. Locate the corresponding resistance/voltage targets in the Thermistor Reading Chart.

Step 2 — Thermistor Resistance Test (Power OFF)

Purpose: Confirm the sensor element is healthy.

  1. Power OFF the unit.

  2. Unplug the thermistor from the PCB.

  3. Set the multimeter to Ω (resistance).

  4. Access the thermistor clip: flip it over to expose the metal strips. If probe tips are too thick, wrap a short piece of thin wire or a paperclip around each probe to make contact with the strips.

  5. Measure the resistance across the two thermistor pins.

  6. Compare the value to the Thermistor Reading Chart target for the measured ambient.
    Pass: Reading matches chart (± typical tolerance).
    Fail: Reading is open/short or significantly off—replace the thermistor.

Step 3 — PCB DC Voltage Supply Test (Power ON)

Purpose: Confirm the board is supplying correct DC to the thermistor circuit.

  1. Reconnect the thermistor to the PCB.

  2. Restore power.

  3. Set the multimeter to V DC.

  4. Use a solid DC neutral (reference). The black wire in the fan-motor plug is a suitable DC neutral.

  5. Probe the two pins feeding the thermistor (from the top of the board with the plug inserted or from the bottom of the PCB, as access allows).

    • One pin should read approximately +5 V DC (board supply).

    • The other pin will show a temperature-dependent DC voltage that correlates with ambient.

  6. Compare to the Thermistor Reading Chart expectations.
    Pass: ~5 V DC present on one pin and temperature-correlated voltage on the other.
    Fail: Missing/incorrect 5 V DC or nonsense readings—likely PCB fault; replace PCB after confirming wiring and connectors.

Quick Decision Flow

  • Resistance out of range (power OFF) → Replace thermistor.

  • Resistance OK, but PCB test fails (no ~5 V DC or incorrect temperature voltage) → Replace PCB.

Tips & Common Pitfalls

  • Always verify ambient temperature first; it frames both resistance and DC voltage expectations.

  • Poor probe contact at the clip is a frequent source of bad readings—use thin wire/paperclip if needed.

  • Use DC for supply checks; AC ranges will mislead and can mask PCB issues.

FAQs

Q: Can I test with the thermistor still plugged in during resistance checks?
A: No. For a valid resistance measurement, the thermistor must be unplugged from the PCB.

Q: What if I can’t reach the pins from the top of the board?
A: You can test from the bottom side of the PCB with the thermistor plugged in, using the DC neutral reference.

Q: What’s the reference voltage I should expect?
A: One pin at approximately +5 V DC; the other varies with temperature per the Thermistor Reading Chart.