How to Test a Mini-Split Thermistor (Temperature Sensor)

Sensor faults are some of the most common mini-split error codes: room air, indoor coil, outdoor coil, outdoor ambient and compressor discharge sensors all report to the boards. Most are NTC thermistors, meaning their resistance drops as temperature rises.

Safety: Turn off power at the disconnect before unplugging sensors. On outdoor units, wait for the inverter capacitors to discharge (see working safely on inverter boards).

Common codes that point here

Each brand names its sensors differently, so check the code chart for the unit:

• Midea-platform units (Senville, MRCOOL): T1 room, T2 indoor coil, T3 outdoor coil, T4 outdoor ambient, TP discharge.
• Gree: F1 indoor ambient, F2 indoor evaporator, F3 outdoor ambient, F4 outdoor condenser, F5 discharge.
• Daikin: C4, C9, H9, J3, J6.
• Mitsubishi MSZ-GL: 1 blink (room), 2 blinks (indoor coil).

See the mini-split error code lookup.

Step 1: Find the right resistance table

Thermistors come in different values (5k, 10k, 15k, 20k and 50k ohms at 77°F are all used), and the same unit can use different values for different sensors. The service manual has a table of resistance at each temperature for every sensor. Don't guess from another brand's table.

Step 2: Measure

1. Power off and unplug the sensor's connector from the board.
2. Measure resistance across the two connector pins.
3. Measure the actual temperature at the sensor bulb (or let it sit at room temperature and measure the air).
4. Compare with the table.

What the reading tells you

• Open / OL: broken sensor or wire. Replace the sensor.
• Near 0 ohms: shorted sensor or pinched wire. Replace.
• Far off the table: failing sensor. Replace.
• Matches the table: the sensor is probably fine. Check the connector for corrosion or a loose pin, then the board.

Step 3: Check the wire and the mounting

Wiggle the lead while watching the meter; a reading that jumps means a broken conductor inside the jacket. Coil sensors must be clipped tightly in their well or on the pipe; a loose sensor reads wrong even when it's good.

Step 4: Board check

With the sensor plugged in and power on, many boards put a low DC reference voltage on the sensor circuit. If a good sensor is installed and the code persists after a power reset, the board's sensor input is the likely problem.

Reading thermistor values

Most mini-split thermistors are NTC (negative temperature coefficient) sensors: resistance goes down as temperature goes up. Common room and coil sensors are often rated around 10 kΩ or 20 kΩ at 77°F (25°C), and discharge sensors are often around 50 kΩ at 77°F, but always use the table for that model.

Make the test meaningful

• Measure the actual temperature at the sensor with a thermometer or thermocouple, then look up the expected resistance. A sensor that reads perfectly at room temperature can still be wrong on the coil.
• Test at two temperatures. Warm the sensor in your hand or a cup of water (protect the connector) and confirm the resistance drops smoothly.
• Check the voltage at the board with the sensor plugged in, if the service manual lists it. Some manuals give expected voltage at the connector by temperature.

Where each sensor lives and what it affects

• Room (T1): controls when the compressor ramps up and down. A bad one causes over- or under-cooling.
• Indoor coil (T2): freeze protection in cooling, anti-cold-air and overheating protection in heating.
• Outdoor coil (T3): defrost decisions and condenser high-temperature protection.
• Outdoor ambient (T4): sets operating limits and compressor speed ranges.
• Discharge (TP): protects the compressor from overheating. A bad TP sensor can trigger false high-discharge-temperature shutdowns.

Common mistakes

• Testing with the sensor plugged in. Unplug it first, or you'll read the board in parallel.
• Using a generic 10 kΩ chart for every brand. Curves differ.
• Missing the clip. A sensor that's loose on its tube reads air temperature, not pipe temperature, and causes bad control without any error code. Re-seat it and insulate it if the original was insulated.
• Replacing the board for a sensor code without checking the sensor and its harness.

FAQ

Sources

• Mitsubishi Electric MSZ-GL Indoor Unit Service Manual OBH732
• Gree All-Match R32 Service Manual (error codes and sensor resistance)
• Daikin Service Manual SiUS091601ED