Why does Prime switching modes when cold?

That's quite normal. The gas-engine supplements heat when it's really cold, allowing EV by using warmed coolant for heating the cabin. When depleted, the engine runs for a bit then shuts off again.

Here is an example from mine doing exactly that last winter.

 

I don't know the rhyme or reason but yes it will do that in colder weather. I can go 18 miles+ on EV only when it's over 20°f but my wife will have the ICE kicking in after 5-10 miles on the same route with the same temperature. I'm thinking it's got something to do with driving style and cabin heat settings.

 

I have not confirmed it yet but speculate that the ICE will come on to achieve the climate control temp setting you have selected when the heat pump is unable to on its own.


iPhone ?

 

try turning the heat all the way down to low, and turn the fan off

 

I've posted this in many threads. But PP has a mind of its own when to turn on ICE and off. In general, the colder the temp the more prone for the ICE to fire. And, once ICE fires and completes the warm-up cycle, it is more prone to re-fire for the subsequent drive

 

When the battery is cold - whatever the car decides is 'cold' - the ICE will start if you call for lots of power from the battery. So with low speed or a light touch on the accelerator, you might be able to run on the battery. But with higher speed or a heavy foot, the ICE will start.

 

At -20C, it needs a VERY light touch indeed (haven’t quite been able to do it yet, not for lack of trying).

 

The heat pump will not provide sufficient heat, or any heat at all, below certain temperatures. It won't heat the cabin nor defrost the windshield. Only the engine will get that done when below these temperatures. My older home heat pump below about 34 to 35F spends more time in defrost than in heat. That's when the electric resistance heat elements switch on. In the Prime the gasoline engine starts when the heat pump is inadequate.

 

This idea is not going to ring well for many, but if you really want to drive EV in "extreme" cold this formula worked for me. And it needs to be done with the heater OFF, windows open if necessary because they're getting foggy
The simple part is, If the ICE fires. you/re going to fast ! ie: if in extreme cold the ICE fires at the same spot in a routine trip, it's (the car) has pasted a point of it's programming tipping point for EV only propulsion, and going slower up and until that point will sometimes let the driver stay in EV mode. If you still get the ICE firing at the same point, you have to go even slower, up and until that point. The EV System(s), need to warm up too, and there's a lot of COLD in the system. that needs to warmup from use in Extreme Cold before that EV system will allow the electric systems to work solo, without the ICE adding waste heat for (it, the EV system(s)) to use.

It gets more complex when figuring in the full daily or typical drive. There are a ton of variables that change how the Prius operated due to typical drive that it does it's best to be most efficient with.
So what works for one Prius driver, doesn't always work for others, with a unique daily drive scenario. And that's the biggest part most drivers don't get for a long time.

LIke I mentioned above, this is not going to appeal to the common driver who just wants to drive the car and have car do as its commanded.
see @EdPalmer42 post #7 above.

Please Don't Forget, that running in EV mode in extreme cold by bypassing the Prius efficiency programming can and will reduce the cars Electrical Systems life cycle, much in the same way that running the Prius too HOT will.

 

My Prime's ICE kicks on in "EV" mode if I use the rear window defroster heated wire gizmo, but not if I use lots of cabin heater.