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Hey, this may not be unique to the prime, and the answer may be hidden somewhere in the massive owner's manual. My mom's 2015 forester does similar things so this may just be how modern car climate control system work.

Whenever I put the climate control on in full auto, the AC always turns on, regardless of whether I'm heating or cooling the car. I also get cool air out of the vents sometimes, for example, if I have it set to 70 inside and it's 55 outside, I get a good amount of air coming out of the upper vents that feels cool, not cold but it's not warm toasty air. This is not a matter of it taking a little while for things to warm up, it does this when everything's warmed up. The whole system seems awkward and auto mode seems kind of useless for heating...
 

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2017 RAV4 LE AWD
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If you are using the upper vents (the defrost/window setting) the ac compressor will always cycle. It has been this way for a number of years, my 2007 Corolla did it as well.
 

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Hey, this may not be unique to the prime, and the answer may be hidden somewhere in the massive owner's manual. My mom's 2015 forester does similar things so this may just be how modern car climate control system work.

Whenever I put the climate control on in full auto, the AC always turns on, regardless of whether I'm heating or cooling the car. I also get cool air out of the vents sometimes, for example, if I have it set to 70 inside and it's 55 outside, I get a good amount of air coming out of the upper vents that feels cool, not cold but it's not warm toasty air. This is not a matter of it taking a little while for things to warm up, it does this when everything's warmed up. The whole system seems awkward and auto mode seems kind of useless for heating...
It will act differently depending on the outside temp and humidity. You're better off just ignoring what it is or isn't doing and just adjust the temp to suit your wishes. If you're too warm turn the temp down, etc. And yes this is all covered in the manual but bottom line is to leave it alone and just turn the temp dial up or down as you need to. Apart from the "S-Flow" dysfunction its a pretty smart system. (I recommend leaving S-Flow turned off.) And don't assume that because the little A/C light is on the the compressor is running because mostly it isn't.
 

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Newer cars work this way, there doesn't seem to be a blast the heat or cool air. I do miss that, on a freezing day having a blast of really hot air feels good.
 

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2021 Rav4 Prime, SE / 2018 Outlander PHEV
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The Auto setting does make some interesting choices. It looks like "S-Flow" checks the seat sensors and won't bother with the rear seats (or even passenger side front) if it doesn't find anyone there. There is also a humidity sensor for the windshield that will automatically turn on the defog/windshield air (and associated AC). We haven't seen any really low temperatures here in Utah yet so I've been content to let Auto do its thing, but I suspect I'll over ride it once things get properly winter like. I hope the heat pump option will be more efficient than the resistance based heating element on our Mitsubishi Outlander PHEV. That system would only try to keep up with a 20 degree temperature difference (ask for 65 when it was 40 outside and the ICE would kick on). Toyota says the heat pump system will work down to 14 degrees F.
 

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A/C's are programmed to be on in automatic climate controlled systems unless the ambient temperature is less than 40F. This is because most climates have moisture in the air, and the A/C will remove some of that moisture before heating/cooling you off to prevent windows from fogging/general comfort. If you live in a dry climate like me, you don't need the A/C to operate when the vehicle is in heating mode, so you can just click off the A/C after engaging the auto mode. (Note: I don't have a rav4 prime, just speaking from experience on other vehicles).

If it makes you feel better, leaving it on won't consume much extra energy. A/C's can operate very efficiently when the temps outside are only 50 degrees. Your condenser fan likely uses more energy running than the compressor itself.
 

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I should append that post, I just realized that the Prime doesn't always have ICE heat... So it may not run the A/C in cooling mode if it's brisk out (50F-ish). In a normal car HVAC setup, the A/C would cool the incoming air to remove humidity, then the radiator fluid would reheat to desired temperature determined by the system. Without a radiator loop always on, I don't think the Prime has any method to reheat after cooling unless the engine is on, and could therefore only heat (using the A/C as a reverse heat pump) if say it was only 50F out and you had the engine off.

As for why the car is cooling when you've set it at 70, and it's 55 out, perhaps the car has been sitting in the sun, and the car thinks that it needs to cool down? My current car can do that sometimes. Another tip on using auto climate controls - ignore whatever temperature is displayed, it rarely reflects the true temperature. This is true for pretty much every car out there, as it's impossible to monitor all variables impacting temperature felt in the drivers seat. Instead, think of the temperature control as just a number that goes up or down. If your too cold, bump the digits up a few degrees, if too hot, do the opposite. In the winter, I usually need to set my car to 78-85 to feel comfortable, and in the summer, i set it to 69-73, even though the inside of the car never settles anywhere near any of those temperatures (yes, I've measured it).
 
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