This is one of those questions where the answer surprises almost every homeowner. If you have a wall-mounted split aircon in your HDB flat or condo (which is roughly 95% of aircons in Singapore), your unit does not have a thermostat at all. Not in the traditional sense.
That little box on the wall that you can turn to set the temperature in an oven, a heater, or a fridge does not exist on a residential aircon. Your remote does not contain one. The indoor unit’s front cover does not hide one. There is no dial, no mercury bulb, no programmable strip behind any panel.
What your aircon has instead is called a thermistor, and where it sits has a direct effect on how cold your room actually gets, why the unit cycles on and off the way it does, and whether your “set 24°C” actually means 24°C in the room or something completely different.
This guide explains what a thermistor is, exactly where it is on your specific type of unit, why that location matters, what happens when it fails, and what you can do if your aircon is reading the wrong temperature.
Thermostat vs Thermistor: Why the Distinction Matters
The two words sound similar but they describe completely different things.
A thermostat is a control device. It senses temperature and switches something on or off based on a setpoint. The simple round Honeywell unit on a wall in a North American house, or the dial in your oven, those are thermostats. They are both a sensor and a switch in one component.
A thermistor is just a sensor. It is a small electrical component, usually a black or grey bead about the size of a grain of rice, embedded in a plastic housing the shape of a small bullet. The thermistor’s resistance changes as temperature changes. It does not switch anything on or off by itself. It just reports a number to whatever is wired to it.
In a modern split aircon, the thermistor is the temperature reporter. The actual decision-making (whether to run the compressor faster or slower, when to cycle the indoor fan, when to defrost) happens on the PCB (the small circuit board inside the indoor unit). The PCB reads the thermistor every few seconds, compares it to whatever temperature you set on the remote, and sends commands to the compressor and fan.
So when someone asks “where is the thermostat?”, the honest answer is “there isn’t one, but the temperature sensor that does its job is in a specific place, and that place depends on what type of unit you have.”
Where the Thermistor Actually Is on Your Unit
There are three setups in Singapore homes, and each one puts the temperature sensor in a different place.
Setup 1: Standard Wall-Mounted Split with IR Remote
This is the vast majority of HDB and condo aircons. Daikin iSmile, Mitsubishi Starmex, Panasonic XU, Toshiba, LG, Samsung, Hitachi, Midea, Haier wall splits, all use this design.
The thermistor is inside the indoor unit, mounted at the air intake just behind the front grille. When you open the front cover to clean the filter, you are inches away from it (do not touch it). The room air flows past the thermistor on its way into the cooling coil, and the thermistor reads that incoming air temperature. That reading is what the PCB compares against your remote’s setpoint.
The infrared remote you hold in your hand is just a transmitter. Press a button, an IR LED blinks out an encoded signal, the indoor unit receives it. No temperature sensing in the remote whatsoever. The temperature display on the remote screen is showing what you have set, not what the room actually is.
There is also usually a second thermistor in the indoor unit, clipped onto the evaporator coil itself. This one measures the coil temperature (not the room) and is used to prevent the coil from freezing and to modulate fan and compressor speed on inverter units.
Setup 2: Cassette or Ducted Unit with Wired Wall Controller
Common in offices, retail spaces, and some larger condos with cassette aircons in living rooms. The remote is not a handheld IR device but a hard-wired controller mounted on the wall, usually a flat panel with buttons and a small screen.
In this setup, there is a thermistor inside the indoor unit (same as Setup 1), AND there can be a thermistor inside the wired wall controller. The installer chooses which one the system uses by setting a dip-switch on the indoor PCB. Most installers leave it on “return air” by default, which means the indoor unit’s sensor is doing the work. Some commercial settings prefer the wall controller’s sensor because it sits at human height in the occupied zone, not up near the ceiling where the indoor unit is.
If your office or condo aircon seems to overshoot the setpoint and runs colder than it should, this dip-switch setting is one of the first things a technician will check.
Setup 3: Smart Wi-Fi Add-On (Sensibo, Tado, Mitsubishi Kumo, Daikin Onecta)
Increasingly common as homeowners retrofit smart control onto existing aircons. These devices either plug into a wall socket near the aircon or replace the remote entirely, and they communicate with the aircon over IR or Wi-Fi.
The Sensibo and similar devices contain their own thermistor, which sits in the device’s housing on the wall or table. When the device reads “warm enough”, it sends an IR command to the aircon telling it to turn off, or change setpoint, or change mode.
Important: the aircon itself is still using its own internal thermistor for its own decisions. The smart device sits on top, sending commands that simulate what you would press on the original remote. So you actually have two temperature sensors active in this setup, working at different priority levels.
Why the Sensor Location Causes Problems
The biggest practical issue with the standard split design is that the thermistor sits roughly 2 metres above the floor (because that is where the indoor unit is mounted), at the air intake, which is right next to the cold air the unit just produced.
This causes three quirks every Singapore homeowner has experienced without knowing why:
The room feels colder at floor level than at the unit. Cold air sinks. The thermistor reads the warmer air near the ceiling, decides the room is at setpoint, and cycles the compressor down. Meanwhile your feet are freezing on the floor. Setting the aircon to 22°C feels okay at the unit but uncomfortable at sofa level.
The aircon turns off before the room is actually cool. If you have just walked into a hot room and switched on the aircon, the unit’s thermistor reads the warm air being drawn into the intake. As the unit blows cold air, that cold air starts mixing with the warm room air. The mixed air going back into the intake is colder than the floor or far-corner air. The PCB thinks the room is cool enough and reduces output, but the corner of the room near your bed is still warm.
The aircon overcools when something blocks the unit. If a curtain or piece of furniture sits too close to the indoor unit and blocks airflow back to the intake, the thermistor reads stale warm air that never gets refreshed. The PCB keeps running the unit harder, thinking the room is not cooling, when actually you just have a poorly placed sofa.
Premium smart aircons attempt to solve this with extra sensors. Daikin’s smart sensor scans the room for human presence and tries to direct airflow appropriately. Mitsubishi’s i-see Sensor in their Starmex range maps the room temperature and adjusts. But fundamentally, the temperature reading still comes from a thermistor in the indoor unit unless you add an external device.
What Happens When a Thermistor Fails
Thermistors are simple components but they do fail. The most common reasons in Singapore are humidity damage (water vapour eventually corrodes the thermistor leads), physical damage during DIY cleaning attempts, and age (over 8-10 years the resistance can drift outside spec).
The symptoms of a failing thermistor are specific:
Aircon runs constantly without cycling. If the thermistor reads always-warm (a “open circuit” failure mode), the PCB thinks the room never reaches setpoint and keeps the compressor running indefinitely. Your bills climb, the unit overworks, and the room may actually freeze parts of the coil.
Aircon barely runs. If the thermistor reads always-cold (the opposite failure mode), the PCB thinks the room is already at setpoint and shuts the compressor down. You get fan-only operation that does not actually cool.
Wildly inaccurate temperature. A thermistor that has drifted but not fully failed will report values that are off by several degrees. You set 24°C, the room ends up at 27°C or 20°C.
Error code on the unit. Modern aircons will throw an error code when the thermistor reading goes completely out of plausible range. On Daikin units this is often code H6 or H8. On Mitsubishi it might be code 11 or 12. See our brand-specific error code guides for the full list: Daikin, Mitsubishi, Panasonic, Toshiba, LG, Samsung, Haier, Midea.
What the Thermistor Actually Sees Versus What You Feel
This is the part most homeowners find genuinely useful once they understand it. The thermistor reads air temperature at one specific point. Your body experiences temperature based on a combination of air temperature, humidity, air movement, surface temperatures around you, and what you are wearing. The two readings can diverge dramatically.
A practical example. You set the aircon to 24°C on a typical Singapore afternoon. The thermistor at the indoor unit reads 24°C and the PCB throttles the compressor down. But:
- The relative humidity in the room is still 70% because the unit only ran briefly before throttling down
- The walls and floor of your living room are still radiating heat from the earlier hot day
- The sun is hitting the west-facing window and pumping radiant heat into the room
- You just got back from outside and your body is still hot from walking in 32°C heat
The air at the indoor unit reads 24°C, technically meeting your setpoint, but you still feel warm. This is not the aircon being broken. It is the limitation of a single sensor located at the ceiling making decisions about a complex room.
Premium models address this in different ways. Some Daikin Smart units use infrared room scanning to detect human presence and adjust airflow direction. Some Mitsubishi Starmex models with the i-see sensor scan the room every few seconds and bias the temperature reading toward where people are sitting. Panasonic’s nanoe and ECONAVI systems do similar work. These features genuinely help in large rooms, but they are not magic. The underlying limitation (one or two thermistors trying to represent a whole room) still applies.
For most Singapore homes the simplest answer is to set the aircon 1-2 degrees cooler than the temperature you actually want to feel. If you want 25°C at sofa level, set 23 or 24°C on the remote. After a few weeks you will know your specific room’s offset.
How Much Does It Cost to Fix?
Thermistor replacement is one of the cheaper aircon repairs because the part itself is small and the labour is straightforward. Typical Singapore pricing:
- Indoor thermistor (room air sensor or coil sensor): $90 to $160 including parts and labour
- Outdoor thermistor (ambient or discharge sensor): $100 to $180 including parts and labour
- Diagnostic visit if you are unsure whether the thermistor is the actual fault: $40 to $80, usually credited against the repair if you proceed
The price varies because thermistors are brand-specific and some are easier to source than others. Daikin and Mitsubishi parts are widely stocked in Singapore. Older Toshiba or Carrier parts can require ordering and add a few days to the lead time.
A note on warning signs: if a technician quotes you over $250 for a single thermistor replacement, get a second opinion. The component itself costs the supplier under $30 wholesale. Anything over $200 is either including unrelated work, marking up parts, or quoting for premium-brand sensors. Ask for the breakdown.
Can You Replace a Thermistor Yourself?
Technically the swap is simple. The thermistor unplugs from the PCB connector and the new one plugs in. No refrigerant work, no electrical hazard if the unit is properly isolated.
In practice we strongly recommend against DIY replacement for three reasons:
First, you need the right part. Thermistors are not generic. A 10k ohm thermistor and a 50k ohm thermistor look identical but are not interchangeable. Using the wrong one means the PCB reads wrong values and the unit behaves erratically. Each brand and often each model uses a specific resistance value.
Second, accessing the thermistor usually requires removing the indoor unit front cover and sometimes the blower assembly. The clips are different on every brand. Cracking a cover during DIY adds an unnecessary cost to the repair.
Third, if the unit is still under manufacturer warranty (typically 5 years for the compressor, 1 year for parts), DIY work voids it. A $30 saving on labour can cost $600 if the compressor fails six months later and Daikin refuses to honour the warranty.
What You Can Do to Get a More Accurate Reading
Even with a perfectly functioning thermistor, the indoor unit’s location means the reading is not the same as the room temperature at sofa level. Three practical things help.
Use a separate room thermometer. A cheap digital thermometer or a smart home sensor placed at sofa level tells you the actual temperature where you are. Compare it to what you set on the aircon and you will quickly learn what “23°C on the remote” actually means in your room (often 25-26°C at floor level).
Make sure the indoor unit can breathe. Curtains, furniture, decorations, or shelves within 50cm of the indoor unit affect the thermistor reading. Keep the area around the air intake clear.
Consider a smart remote with external sensing. Sensibo and similar devices solve the location problem by placing the temperature sensor where you actually sit, not where the aircon is mounted. The cost is around $150 to $200 for the device, and you get app control plus more accurate temperature management. If you have been frustrated with your aircon never quite getting to the temperature you set, this is the cheapest single fix.
Have the unit serviced. A clogged filter or biofilm-coated thermistor reads wrong even when the part is mechanically fine. Routine servicing keeps the sensor exposed to clean airflow and reading accurately. See our guide on how often to service your aircon.
The Quick Answers
For homeowners just looking for the short version:
Where is the thermostat on my aircon? There isn’t one. There is a thermistor (a temperature sensor) inside the indoor unit, at the air intake just behind the front grille.
Is the thermostat in the remote? No. The IR remote is just a transmitter. There is no temperature sensor in it. The temperature on the screen is what you set, not what the room is.
Why does the room not match the temperature I set? Because the sensor is at the indoor unit at ceiling height, not at your sofa. Cold air sinks, so the floor is colder than the unit reads.
Can I move the temperature sensor? Not on a standard split with IR remote. You can add a smart remote (Sensibo) that places a sensor wherever you want.
How do I know if my thermistor is faulty? The aircon runs constantly without cycling, barely runs at all, reports impossible temperatures, or throws a sensor-related error code.
How much to replace a thermistor in Singapore? $90 to $160 for indoor sensors, $100 to $180 for outdoor sensors, including parts and labour.
Need Help With an Aircon Sensor Problem?
If your aircon is not cooling properly, cycling strangely, or showing a sensor-related error code, we can diagnose whether it is actually a thermistor problem or something else. Same-day appointments across Singapore, all major brands. We will test the sensor with a multimeter, confirm the actual fault, and quote the real fix.
WhatsApp us at +65 8818 5781 or book online at lioncityaircon.sg/booking. If your unit is showing an error code, send us a photo and we can often tell you the cause before we arrive.
