Engine started to protect system ev driving unavailable

[YarisHybrid2016]

I'm seeing: 8000 miles, cold temps, most driving EV only.

Seems the car wants to run the ICE for maintenance/life reasons.

[TonyHSD]

Oil pressure sensor likely not or the slope parking. More likely been Battery temperature or soc and eventually outside temperature and engine been conditioned for standby. It’s one thing to start the engine at 12C° and completely different at -2C°, the oil viscosity will be very different. What else can it be. Hev Toyota often refuse to go in ev mode when Battery is too low or too full. Another time when they refuse to stay in ev mode selected via the ev button is when it’s freezing cold, -4C° and the car says ev mode not available and fired the ice. Only Toyota engineers knows what exactly is the reason, until official information been leaked we can only guess what happens. 👍

[Kristin A]

Started getting this message a few weeks ago. Seems to be happening when car is switched to hybrid with cruise control on. Taking it to dealer because it is happening consistently. I use hybrid on the longer highway stretches and can’t be pulling over to restart the car to get the EV back on!

[Nick72]

Used to get this issue with EV mode unavailable all of the time on my Peugeot 3008 300GT Plug In Hybrid4. Like, every other drive. No obvious reasons. It got sent back and I switched to the R4P.

In the R4P I don't think I've seen this message but my memory is bad these days. It does however switch on the ICE when I'm in EV mode from time to time. Not often. But just the usual thing to circulate fluids etc to keep the ICE from sustaining any damage. Like maintenance really.

But if folks are getting this frequently and getting the warning message it does feel like something is wrong. Faulty sensor somewhere or a failing subsystem?

Or, there's some common circumstances like, it's very cold, you're towing, going above 80 something or whatever mph EV limit is, or something else like that which would make the car need to switch out of EV mode. 

Disappointing thing is as cars get more complicated, the user needs to know the why. But why did you switch modes. I'm not talking nth level of details, just enough for you to know it's normal or abnormal.