Thermostat Accuracy

The thermostat is attached to the cooling loop and regulates the flow of coolant through the radiator. I think you are talking about the outdoor thermometer. The sensor for that is located behind the front bumper on the right side (facing forward).

Tom

 

It's located pretty much as far forward as possible (in front of the radiator and all) to avoid falsely measuring any heat from the engine, and should be reasonably accurate particularly when the car is moving forward as the airflow should be moving the engine's heat away from the thermometer.

However, the road (being black and all) tends to absorb heat from the sun and I suspect that the fact that the thermometer is only a couple of feet above the surface of the hot road affects it's reading.

I'm not sure bank thermometers are known for their accuracy, but if yours is consistantly reading high (particularly at night when the sun on the road isn't a factor) then you might want to talk to your dealer and see if they have a way to calibrate it.

 

Agreed. I've also noticed that my car exterior thermometers (all cars I've owned, not just Prius) read several degrees higher than the temp shown on various thermometers I pass on the road. The difference is greater on particularly hot days (5+ degrees).

 

Another thing to note is that the car reads the outdoor temperature in Celcius and then converts it to Fahrenheit. By doing this it can cause the reading to be off by 1 degree either way. For example if the car reads 0 degrees C that is 32 degrees. If the car reads 1 degree C that is 34 degrees. So if it is 33 degrees outside your car will read either 32 or 34. I personally find this annoying but I don't think most people care.

This doesn't account for the car being off by 5 degrees but as someone else said bank thermometers are not very good to use as a reference. I drive by one on the way to work and it is off by about 7 degrees. And as someone also said roads absorb heat on sunny days and radiate it upward so car thermometers will not read the same as a thermometer over a grassy surface.

I have an accurate thermometer at my house and comparing my car to it they are usually within 1 to 2 degrees so my Gen III is about as close as you can expect.

 

I think it's always hotter on the pavement than on the grass.

 

There's a dealer changeable parameter for the ambient (external) thermometer offset, so you can have the "outdoor temp" read up to 3C degrees hotter or colder than what the sensor is actually reading.

The parameter is called the: Ambient Temperature Shift. The default value is "Normal". The param description is: "Function to control the shifted ambient temperature in relation to the displayed ambient temperature." Legal values are: +3 C, +2 C, +1 C, Normal, -1 C, -2 C or -3 C, and it's part of the "A/C amplifier" ECU.

 

I would not trust the signs at banks and such either. They can vary pretty badly too. It all depends on where the sensor is located.