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Car: 2010 Mazda 3 2.5L Manual

My car's outside temperature display is way off. It is showing warmer than the actual temperature. For example: Today the real temperature was -9C and it showed +18C.

User manual for the car says the following: Under the following conditions, the ambient temperature display may differ from the actual ambient temperature depending on the surroundings and vehicle conditions:

  • Significantly cold or hot temperatures.
  • Sudden changes in ambient temperature.
  • The vehicle is parked.
  • The vehicle is driven at low speeds.

so the question is; whether the outside temperature sensor is tied to HVAC system of the car?

I don't really care about this as long as it doesn't affect the HVAC system. I don't notice any issue with the HVAC system right now. However if it does affect heating/cooling, I will try to locate the sensor and clean/replace it.

EDIT: My car does have automatic climate control. There are two conflicting answers. One saying it does not affect the climate control system and one stating it does. Does anyone have a source to confirm this? It is also possible this is different from one car to a another.

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No, the outside temperature sensor should not affect the HVAC. That sensor is only there to give you the outside temperature reading on your dash. Modern climate control systems use temperature sensor(s) inside the car to control the temperature. It wouldn't make sense for the system to be metering from outside temperature if it's below freezing outside, but the cabin has already been warmed to room temperature!

To address the snippet from your user manual, the reason for the temperature reading to be off would be for the following reasons:

  • The sensor has a limited temperature range in which it's accurate
  • The temperature display may be using some sort of average over time which may not reflect sudden changes in temperature
  • The sensor may be affected by heat from the engine or the road when driving at low speeds because it's located in the front of the car

To address Al_'s response, here is the wiring diagram for the automatic climate control HVAC system from the 2010 Mazda 3 service manual (page 07-03A-19):

HVAC SYSTEM WIRING DIAGRAM

From this diagram we can see that there is actually a dedicated evaporator temperature sensor.

According to autoacrepair1.com on the general function of an evaporator temperature sensor:

The evaporator temperature sensor tells the PCM or ECM the temperature of the evaporator and keeps the evaporator core from freezing. [...] The evaporator temperature sensor acts as a protection device maintaining the evaporator temperature between 34º F and 37º F. [...] When the temperature drops bellow a preset temperature value the compressor magnetic clutch will be turned off, thus stopping the compressor and avoiding damage to it

And on page 07-11-12 there is an assembly note for installing the evaporator temperature sensor.

Finally, you'll notice on the wiring diagram that the ambient/outside temperature sensor is completely absent from it. To prove that this sensor is indeed different from the evaporator temp sensor, here is the diagram showing installation/removal of the ambient temp sensor (from page 07-40A-22). So rest easy, you won't need to replace it for now. If you want to replace your evaporator temperature sensor, then by all means, the assembly note will show you where it goes.

  • Good answer ... I've also seen where the sensor may be affected by direct sunlight on the hood (bonnet). My truck is affected like this. When I start the truck up, it shows a temp far in excess of what it is outside, then within a few minutes of being started and going down the road, it show a more accurate reading. There's lots of factors, but I believe you are correct it and the HVAC system don't meet. – Pᴀᴜʟsᴛᴇʀ2 Dec 7 '18 at 17:48
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    In that case then there's no problem at all, but remember, not cycling the compressor at all below a certain ambient temperature is always better than cycling it on and off it very rapidly to keep the evaporator from freezing because of a low ambient temperature. Makes the compressor clutch last longer. – Al_ Dec 14 '18 at 22:42
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In an automotive HVAC system, the sole purposes of the external temperature sensor are:

1)to allow an ATC (automatic temperature control) system to automatically evaluate a comfortable internal temperature setpoint without any operator intervention.

2)to prevent compressor operation below a certain ambient temperature (avoids liquid refrigerant compressor slugging), especially so when dealing with a fixed displacement compressor, definitely vulnerable to refrigerant slugging (liquid refrigerant returning to the suction port and destroying everything inside).

Your real worry should lie in the second fact.

The temperature reading looks way off to be considered as normal sensor operation.

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Yes, your auto climate control uses an ambient temp sensor which is located in front of the coolant radiator outside the car. The location is why it is sometimes inaccurate for outside temp readings displayed on the dash but is optimal for the AC as it knows the exact temp of the air coming across the AC condenser.

enter image description here

Number 9 in the image below

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  • In the diagram, where does it actually show the ambient air temp sensor connecting to the climate control unit? It connects to the BCM. It's not showing the BCM connecting to it either, unless it's in some other diagram? I'm not saying it is or isn't as you say, I'm just not seeing the connection. I'd also suspect different auto manufacturers will do it differently, where the ambient air temp sensor may have some influence over the HVAC and in others it doesn't. – Pᴀᴜʟsᴛᴇʀ2 Dec 14 '18 at 22:41
  • Image does not show it but the climate control is connected to the BCM, modern cars share sensor and other data across the system can bus and is not always reflected as an actual electrical connection, as all the control modules are networked on the can bus. – Moab Dec 14 '18 at 22:43

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