---
# System prepended metadata

title: What should I do if the air conditioning temperature sensor fails? How to fix it?
tags: [Sensor]

---

In most air-conditioners (mini-split, window AC, HVAC controllers), the “[temperature sensor](https://www.onzuu.com/category/temperature-sensors)” is a NTC thermistor: its resistance drops as temperature rises. Key parameters are usually R25 (resistance at 25 °C) and B value (curve steepness). 
![ac-problems](https://hackmd.io/_uploads/rkHoJv9Ebg.jpg)

Below is a practical, “do-this-first” troubleshooting + repair guide, and then real [sensor](https://www.ampheo.com/c/sensors) examples.

**1) Safety first**

* Turn off power to the AC (breaker/disconnect) before touching wiring or the indoor unit PCB.
* Avoid poking around near mains terminals and capacitors unless you’re trained.

**2) What failure looks like**

A bad AC temperature sensor often causes:

* Wrong temperature reading / weird behavior (runs nonstop or short-cycles)
* Coil icing (if the evaporator/coil sensor fails)
* Error code indicating “sensor open/short” (varies by brand/model)

**3) Step-by-step diagnosis (works for most AC thermistors)**
**Step A — Identify which temperature sensor failed**

Common AC thermistors:

* Room/return-air sensor (controls comfort temperature)
* Evaporator/indoor coil sensor (prevents freeze-up, controls superheat logic in some systems)
* Outdoor ambient / outdoor coil sensor

Where it is matters because:

* A room sensor failure affects setpoint control.
* A coil sensor failure often leads to icing or protective shutdown.

**Step B — Visual & connector check (fast wins)**

* Unplug/reseat the sensor [connector](https://www.onzuu.com/category/connector-interconnects) on the control board.
* Inspect for:
  * broken wire near the sensor head
  * corrosion/green copper at connector
  * pinched cable (very common where covers clamp harnesses)
* Make sure the coil sensor is properly clipped to the coil/tube (bad contact can look like a “bad sensor”).

**Step C — Ohmmeter test (the most reliable check)**

1. Disconnect the sensor from the PCB.
2. Measure resistance across the two sensor leads.

**Interpretation:**

* Open circuit (∞ Ω / OL): sensor or wire is broken → controller often thinks it’s extremely cold or extremely hot (depends on design).
* Short circuit (~0 Ω): sensor or wire shorted → controller reads an extreme temperature.
* “Drift” (wrong but not open/short): aging/moisture → wrong temperature reading.

**Step D — Compare resistance to a known curve/table**

You must compare against the correct thermistor type (10k, 15k, 20k, 50k, 100k… and the curve). Many manufacturers publish R–T tables for their units.

Example: some AC documentation provides tables for 15k / 20k / 50k thermistors (values are very different from 10k). 

So: don’t replace a 20k with a 10k even if it “fits”—it will read the wrong temperature.

**4) How to fix it (from easiest to “replace”)**
**Fix 1 — Connector/wiring repair**

* Clean contacts (electronics contact cleaner), re-crimp or re-pin if loose.
* Repair broken wire close to the sensor head (strain relief it afterward).

**Fix 2 — Mounting/thermal contact correction**

For coil sensors:

* Ensure it’s tightly clamped to the correct tube/coil location.
* Add/replace insulation foam over the sensor if the design uses it (improves accuracy, reduces false readings).

**Fix 3 — Replace the sensor (most common final fix)**

When replacing, match:

* Thermistor curve (R25 + B value or exact “Type” table)
* Temperature range & moisture protection (AC environments are humid)
* Mechanical form (probe/bead, epoxy, clip style, connector)

**5) Actual sensor examples (real parts + what the numbers should look like)**
**Example A — “10K NTC Type II” commonly used in HVAC controls**

Many HVAC wall/duct sensors are 10k NTC Type II, often specified around R25 ≈ 10kΩ with a B value near ~3975K. 

From a Type II table (example values):

* 10 °C → ~19.9 kΩ
* 21.1 °C → ~11.9 kΩ
* 26.7 °C → ~9.3 kΩ 

So if your room is ~25 °C and you measure something like ~9–11 kΩ, that’s plausible for a 10k Type II. If you measure 50 kΩ at room temp, you likely have a different thermistor type (e.g., 50k or 100k family).

**Drop-in thermistor component example (10k class):**

[Vishay](https://www.ampheo.com/manufacturer/vishay) [NTCLE100E3103JB0](https://www.ampheo.com/product/ntcle100e3103jb0-26769931) — a 10k NTC with B parameter around 3977 K (commonly used curve family). 

**Example B — 10k NTC (B≈3950) “generic electronics” curve**

A very common electronics curve is 10k @ 25°C with B≈3950; a typical lookup table shows:

* 0 °C ≈ 31.77 kΩ
* 25 °C = 10 kΩ
* ~40 °C ≈ 5.33 kΩ 

If your AC uses this style and you read wildly different values at known temperatures, it’s a strong indicator of failure or wrong sensor type.

**Example C — 100k NTC (also common in some designs)**

Some systems use 100k @ 25°C thermistors.

[TDK](https://www.ampheo.com/manufacturer/tdk-corporation)/EPCOS [B57891M0104J000](https://www.ampheo.com/product/b57891m0104j000-26768667) is 100kΩ @ 25°C with a typical B(25/100) ≈ 4450K. 

If you accidentally replace a 100k sensor with 10k, the AC will “think” it’s at a totally different temperature and control will be wrong.

**Example D — Board-level thermistor (used inside electronics modules)**

If you’re repairing a control PCB (not the probe sensor), you may see SMD NTCs like:

[Murata](https://www.ampheo.com/manufacturer/murata-manufacturing) [NCP18XH103F03RB](https://www.ampheo.com/product/ncp18xh103f03rb-26760515) — 10kΩ @ 25°C, B constant around 3380K. 

**6) Quick decision checklist**

* OL/∞ Ω → broken [sensor](https://www.ampheoelec.de/c/sensors) or cable → replace/repair wiring
* ~0 Ω → shorted sensor/cable → inspect for pinch/water ingress → replace
* Resistance “reasonable” but control is wrong → likely wrong curve/type, bad mounting contact, or intermittent connector