Room Won't Reach Set Temperature? Thermostat Sensor Problem

You set your aircond to 24°C. It runs for hours. The room feels warm. But the unit doesn't seem to be trying harder — it cycles on and off as if it thinks the room is already cold enough. The problem might not be low gas, dirty coils, or a weak compressor. It could be a faulty thermostat sensor.

What Is the Thermostat Sensor?

Your aircond has a small temperature sensor called a thermistor — a tiny component that measures room air temperature and reports it to the PCB (circuit board). The PCB uses this reading to decide whether to run the compressor harder, slow it down, or cycle it off.

If the thermistor reads incorrectly — say it tells the PCB the room is 23°C when it's actually 27°C — the aircond "thinks" it's done its job and reduces cooling output. You feel warm, but the aircond is happily idling because its sensor says everything is fine.

Signs of a Faulty Thermistor

• Room never reaches set temperature: The aircond runs but the room stays 3-5°C warmer than what you've set.
• Aircond cycles too frequently: The compressor turns on and off every few minutes instead of running for sustained periods.
• Aircond runs non-stop without cycling: The opposite — the sensor reads too high, so the compressor never turns off, leading to ice formation on the coil.
• Temperature display on remote doesn't match reality: Some airconds show the current room temperature. If it shows 23°C when the room clearly feels like 28°C, the sensor is lying.
• Overcooling: The room gets much colder than the set temperature because the sensor reads warm when the room is already cold.

Many of these symptoms overlap with other issues. For a broader list, see our guide on why your aircond might not be cold.

Where Is the Thermistor Located?

Most wall-mounted split units have the room temperature thermistor attached to the evaporator coil area or mounted near the air intake grille. It's a tiny component — usually a small bead on two thin wires. There's often a second thermistor on the evaporator coil itself (coil temperature sensor) that serves a different purpose (anti-freeze protection).

Can You Test It Yourself?

If you have a multimeter, you can test the thermistor's resistance. At room temperature (25°C), most aircond thermistors should read around 5-15 kΩ (varies by model). If it reads open circuit (infinite resistance) or very low (near zero), it's faulty.

However, the more common failure mode is a thermistor that's not completely dead but reads inaccurately — say it reads 20°C when the room is actually 27°C. This is harder to diagnose without comparing its reading to a known-accurate thermometer.

How Much Does Thermistor Replacement Cost?

The thermistor itself is one of the cheapest aircond parts — typically RM10-30 for the component. The labour to diagnose, access, and replace it runs RM50-100. Total cost: RM80-150 — one of the most affordable aircond repairs.

Compare this to what happens if you ignore it: an aircond running with incorrect temperature sensing wastes electricity (running too much or too little), causes premature compressor wear, and can lead to ice buildup on the evaporator coil that eventually causes water leakage.

Other Causes of "Room Won't Reach Temperature"

Before blaming the sensor, rule out these common issues:

• Dirty filters/coils: Reduced airflow means reduced cooling output
• Low gas: The most common cause of poor cooling
• Undersized unit: The aircond simply isn't powerful enough for the room
• Hot weather + poor insulation: On extremely hot days (36°C+), even a properly working unit may struggle
• Blocked outdoor condenser: Can't reject heat properly

If you've ruled out these causes and the aircond still won't reach temperature, the thermistor is a strong suspect. For professional diagnosis in Putrajaya or anywhere in KL/Selangor, our technicians carry replacement sensors for all major brands and can diagnose and fix the issue in a single visit.