Stuck in HV mode

In the previous Prime at least, the car would force HV mode every now and then in order to run the gas engine. You have to run the gas engine every now and then. This should be explained in your owner's manual

 

You could be in ev auto mode. I think the engine has to complete a warm up cycle if front defrost is selected and turned off, but haven’t tried it myself on a 2020.

 

If most of your miles are EV, with few stops at a gas station, I'd say a maintenance mode to exercise the engine and use up gas is likely. Assuming the battery isn't getting too hot for some reason.

Most report EV Auto mostly staying in EV. Though the OP could be driving it hard.

 

Presumably it's not in "HV mode", it's running the engine in EV Mode.

But there shouldn't be much that prompts the engine to start at 50°F (+10°C) in EV Mode, aside from the front defrost, which will basically always do it. Whenever it starts for any reason, it will run until warmed up - it won't stop immediately unless you force it by turning the car off. It's definitely better to let it warm up than to force it off sooner.

Could it be that the battery was considerably colder - below freezing? That could have reduced available output - the battery heater function is intended to avoid that by keeping it above freezing. Engine starting between 0°C (32°F) and -10°C (14°F) will happen sometimes, and will be more common if the battery isn't heated. The EV mode restricts the power output to halfway through the "PWR" bar on the Hybrid System Indicator, but if the battery's cold, that will be more than it's willing to take from it.

Below -10°C, engine start-up is almost inevitable, for cabin heat, and the battery heater gives up somewhere below that temperature. I spent a week in EV Mode with the engine never actually stopping, and hardly any EV range being consumed, but then it was around -20°C (-4°F), with shortish trips.

 

I had to look, he lives in San Diego.

The owners manual gives the operating mode descriptions, my 2020 stays off in ev mode, in Calif where I live. It could be they made a major change in 2023. On mine in ev mode, the gas engine never runs unless front defrost, or going over 84 mph, or battery depleted, that’s it.

 

With heat on (almost no matter what the temps ) there is no one answer.
With the Gen 4 Prime there is a definite correlation between 60 F and when the ICE fires. It's not a big difference but it is there. And that 60 F has to do with the high voltage battery temp, all other conditions being relatively similar and in my testing at those temps with the HAVC systems off, if the HAVC ( heat or Air ) are on the game changes considerably.

 

Front Defrost will turn on the ICE

 

Certain things you do or the way you've got the car setup, can force the car into HV mode. Turning on the front defroster is one of them. Perhaps you've got the car set for performance mode, that would also unexpectedly kick the ICE on also - too much throttle. Whenever the ICE turns on, it's best to let the ECU do it's job and allow the engine to fully warm-up. A manual shut-down/reboot doesn't allow the engine to warm-up and burn-out the moisture, fuel, and contaminates out of your oil. Your oil will sludge up.
I've got my heating system in manual mode and the car in eco mode. I know when the ICE fires, because the throttle response will get sluggish and the MID will indicate engine on. Again, let the ECU do it's job, and allow the engine to fully warm-up. The ECU will allow you to reactive EV mode once the engine is up to operating temperature and the ECU will shut it off.
Read the section of your operating manual; conditions that will fire-up the ICE

 

In Gen 4 Prius Prime, turning the front defogger on in colder weather turns off the BEV mode and turns on the HEV mode. It is interesting that this is the same in Gen 5.

Workaround: Instead of the front defogger, use the A/C with the vents set to windshield (defogger vents) and the fan speed set to medium or high speed, which will defog the interior of the windshield. This saves fuel and battery SOC, too, in comparison to using the front defogger. The only downside is that if the side mirrors are fogged up, it won't defog them.

 

A long up or down slope? That would kick the ICE for assist or engine braking?

 

I was going to ask if you used B mode. Still I can't think of why the engine would come on in EV mode after a 2 mile decent, unless.
Many Gen 4 owners talked about how their engine would fire, after a full charge at the top of a hill, while going down the hill.
The general answer was that the engine turned on to dissipate regen to the fully charged battery.
Whether that is the same or similar to what you are experiencing, idk, but it may, at least, give a few more clues for you to examine.

edit; one way to test if this is what is happening in your case, is to go down that hill with 10 or 20% less EV range when you get to that hill and see if the engine also fires than, given the same termp range and other conditions you've already included above.

 

If the battery's charge level gets high enough from a descent, the car could turn on the engine for compression braking, and to reduce the charge level. Once activated for whatever reason, the engine has to go through its warm up cycle.

 

I know my ICE kicks on for compression braking; if I have the cruise control set and the down hill is too steep for the brakes alone to handle the decent.

 

There are also some weird behaviors when using B mode. Although I've never experienced them in EV mode. But, when manually switching between EV and HV modes there are a lot of time the engine will turn on. Sometimes, it's programming thresholds and other times it due to a combination of driver button and gear changing ( like missing a shift when driving a manual transmission ) .

More recently, I'm noticing B mode starting the engine while in EV mode, if the batteries are cold and wouldn't accept the amount of regen B mode wants to put in the battery, when this would not typically happen when the temp of battery is higher. Using the brake pedal only ( in D, without shifting into B mode ) has never show this behavior of engine on in EV mode for me.