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Now that it is starting to get colder in Rochester, I have noticed the following phenomenon: Within the first 5 minutes or so, if I stop at a red light, the car will sometimes keep the engine on...But I can then get it to turn it off by lowering the temperature setting. In fact, I can sometimes toggle the engine off and on by jumping the temperature setting up and down between, say, 65 and 66 degrees!

So, I guess the Prius makes the decision to keep the ICE on if the temperature you want is higher than it thinks it can provide with the current state of the engine coolant temperature or whatever? Does anyone with more experience with this know exactly what the scoop is. In particular, I was sort of curious whether this depends on what climate control mode you are in. I have tended to keep the climate control out of automatic mode...which means it doesn't automatically turn the air conditioner (in the sense of the compressor to cool the air) on but it does seem to still provide heating if necessary.
 

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Are you talking about a 2004-2005 Prius, or a classic?
The 2004-2005 does have an electric heater in it for when the coolant isn't warm enough, so it's strange it is controllable by the temp control. Not that I am doubting you, just sounds strange.

We had a tad of a cool spell so I think the car ran the heat a bit, but couln't really tell yet. Will have to wait a month or so.
 

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heater...

Same here! I knew the engine provided heat just like a conventional car, but with the added electric heat, too. I guess with 28 degree mornings it takes longer to get the coolant up to temp, before the engine will shut off at a stoplight. I was playing with the heater settings too, but did notice after 10 minutes of driving the engine did shut off at stoplights, but I was still getting heat. Emissions lowered 1st, then the 50 MPG bonus second!
 

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Of course for "emission purposes" the coolant is always kept at a hot temp. So even though the MFD dosen't show it the car will run at idle to keep emmision temp up. Someday we will have an "engine running" light and this will no longer have to be explained.
 
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