Everyone knows what an air conditioner is, but many may not know about its temperature sensor. The automatic start-up and temperature adjustment of an air conditioner are primarily achieved through a crucial component – the temperature sensor. So, how does an air conditioner temperature sensor work? What is it composed of? Let's find out together!
I. How Does an Air Conditioner Temperature Sensor Work? Commonly used air conditioner temperature sensors are negative temperature coefficient thermistors, abbreviated as NTC. Their resistance decreases as temperature increases and increases as temperature decreases. Manufacturers typically use the resistance at 25℃ as the nominal value.

Air conditioner temperature sensors are connected in series with a resistor. A 5V (some air conditioners use +3.3V) voltage is used for voltage division, and the divided voltage is sent to the CPU. Because air conditioner temperature sensors use negative temperature coefficient thermistors, their resistance decreases as temperature increases and increases as temperature decreases. Therefore, the CPU's input voltage follows this pattern: as the temperature rises, the CPU's input voltage increases; as the temperature falls, the CPU's input voltage decreases. This changing voltage is analyzed and processed internally by the CPU to determine the current pipe temperature or room temperature. Through internal programs and manual settings, the CPU controls the air conditioner's operating status.

Because the sampling voltage sent to the CPU varies considerably with temperature, manufacturers typically design it to be half the power supply voltage, based on a 25°C benchmark, to allow sufficient margin for voltage changes caused by temperature variations. If the sampling voltage is designed to be too high or too low, it will not accurately reflect the current temperature changes. This is the working principle of an air conditioner temperature sensor.

II. Composition of an Air Conditioner Temperature Sensor Commonly used NTCs in air conditioners include indoor ambient temperature NTCs, indoor coil NTCs, and outdoor coil NTCs. Inverter air conditioners also use outdoor ambient temperature NTCs, compressor suction and exhaust NTCs, etc. In the circuit, temperature changes cause changes in the NTC's resistance, and the voltage at the CPU terminals changes accordingly. The CPU uses these voltage changes to determine the air conditioner's temperature and thus its operating status.

III. Functions and Common Faults of Various Types of Air Conditioner Temperature Sensors
Common faults of NTCs include increased resistance, open circuit, resistance changes due to moisture and mold, short circuit, poor contact of the plug and socket, or leakage, which can cause abnormal voltage at the air conditioner CPU detection terminals, leading to air conditioner malfunctions. The following analysis first examines the functions of the indoor ambient temperature air conditioner temperature sensor, indoor coil NTC, outdoor coil NTC, exhaust NTC, and intake NTC, and then analyzes common faults of air conditioner temperature sensors based on these functions and principles.

(1) Indoor Ambient Temperature NTC
It is usually installed at the air outlet of the indoor unit's heat exchanger. Its main functions are threefold:
First: Detecting the indoor temperature during cooling or heating to control the compressor's operating time;
Second: Controlling the operating status in automatic operation mode;
Third: Controlling the indoor fan speed.
The indoor ambient temperature NTC automatically starts, stops, or operates at a variable frequency based on the set operating status and the indoor ambient temperature.

Fixed-frequency air conditioners have a temperature difference range of 1℃ between the set value and the ambient temperature. That is, if the cooling setting is 24℃, the compressor will stop when the temperature drops to 23℃ and start again when the temperature rises to 25℃; similarly, if the heating setting is 24℃, the compressor will stop when the temperature rises to 25℃ and start again when the temperature drops to 23℃. It's worth noting that the temperature setting range is generally between 15℃ and 30℃. Therefore, cooling will not work below 15℃, and heating will not work above 30℃.

Variable-frequency air conditioners adjust their speed based on the difference between the set operating temperature and the indoor temperature. The larger the difference, the higher the compressor's operating frequency. Therefore, the compressor speed increases rapidly after startup and then slows down to a lower speed near the set temperature.

(2) Indoor Coil NTC
It is installed on the surface of the indoor heat exchanger and has four main functions:
First: Overcooling protection during cooling;
Second: Overheating protection during heating;
Third: Controlling the speed of the indoor fan motor;
Fourth: Assisting outdoor defrosting during heating.
Indoor coil overcooling protection detection, refrigerant shortage detection, etc.; heating protection against cold air blowing out, overheating protection, etc.

Taking a fixed-speed air conditioner as an example, after the air conditioner has been running in cooling mode for a period of time (e.g., 30 minutes), it automatically checks the temperature of the indoor coil. If the temperature does not reach a certain level (e.g., below +20℃), it automatically diagnoses a refrigerant shortage and activates protection. If the indoor coil temperature drops below +3℃ for some reason, the unit will stop to prevent frost formation (overcooling). During heating, if the indoor coil temperature is below 32℃, the indoor fan will not blow air (to prevent cold air); if it is above 52℃, the outdoor fan will stop; if it is above 58℃, the compressor will stop (overheating). Some air conditioners automatically control the indoor fan speed during heating; some automatically switch to electric auxiliary heating; and some variable frequency air conditioners control the fan speed. Different manufacturers have slightly different designs, but the basic protection functions are similar.

(3) Outdoor Ambient Temperature NTC
The outdoor ambient temperature sensor is installed on the outdoor heat exchanger via a plastic frame. Its main functions are:
First: to detect the outdoor ambient temperature during cooling or heating;
Second: to directly control the outdoor fan speed.

(4) Outdoor Coil NTC
It is installed on the surface of the outdoor heat exchanger and has three main functions:
First: to protect against overheating during cooling;
Second: to protect against freezing during heating;
Third: to control the temperature of the heat exchanger during defrosting.
Heating defrost temperature detection, cooling condensing temperature detection. Heating and defrosting are important functions of heat pumps. For example, the first defrosting is timed by the CPU (usually every 50 minutes), and subsequent defrosting is controlled by the outdoor coil NTC (e.g., defrosting at -11℃, heating at +9℃). The compressor stops when the condensing temperature reaches 68℃, replacing the function of the high-pressure switch; for variable frequency cooling, it reduces the frequency to prevent the coil from continuing to heat up. The ambient temperature NTC controls the outdoor fan speed and preheats the compressor in winter.

(5) Compressor Discharge NTC
It is mainly installed on the compressor discharge pipe and has two functions:
First, it controls the compressor speed by detecting the temperature of the compressor discharge pipe and adjusting the opening degree of the expansion valve;
Second, it is used for overheat protection of the discharge pipe.

It reduces the frequency of the variable frequency compressor to prevent overheating of the outdoor unit, detects refrigerant shortage, and adjusts the opening degree of the electronic expansion valve.

(6) Suction NTC
It controls the refrigerant flow rate, achieved by a stepper motor controlling the throttle valve. Suction NTCs are now less commonly used in household applications.

(7) Fault Analysis
Temperature sensor faults can vary. Indoor and outdoor coil NTCs are subject to constantly changing temperatures, condensation, or high temperatures, resulting in a relatively high failure rate.

The main symptoms include:
(a) The power supply is normal but the unit does not work; it operates for a short period but then stops; the outdoor unit is normal but the indoor fan does not run during heating; the outdoor fan does not work or stops abnormally; the compressor does not start; the inverter effect is poor; the inverter does not work; and defrosting does not occur during heating.

(b) Even when the power supply is normal but the air conditioner is not working, the indoor ambient temperature NTC should be checked; if the air conditioner operates continuously or stops before reaching the set temperature, the indoor ambient temperature NTC should also be checked first; malfunctions in inverter air conditioners can also be related to it. A faulty indoor ambient temperature NTC can cause the CPU to incorrectly determine the indoor ambient temperature, leading to malfunctions.
If the initial diagnosis is a problem with the temperature sensor, the problem can be further confirmed using the sensor temperature-resistance-voltage comparison table provided by different manufacturers. Replace any faulty sensors.