It's not a dumb question. If you can not enter EV mode between 10 - 24 mph then that means you are at Stage 2 or lower. This doesn't make any sense if you've been driving the Prius with the ICE on for longer than 5 miles (in my experience). You might as well check several other things, since it's not just the ICE staying on, but you are noticing MPGs dropping: Coolant / Anti-Freeze Reservoir, there should be reserve fluid Check the Oil Dipstick (low, high or between the marks). Take out dipstick, wipe clean with paper towel, reinsert and check level.
Is anyone sitting on the back seat? If so are they inadvertantly blocking the battery vent? Is the trunk full of stuff all the time? Lastly, do you run the a/c on recirculate all the time which might stop the air pushing thru to the HV battery to cool it?
Update: Didn't do it on the way home last night but did it again this morning for the 3rd time in four days. I'm seeing a pattern here. Always in the morning, after a long while, and it's happened more than once in the same exact spot.....I use ICE to go up the parking ramp at my work then coast to the next up ramp. It's during that coasting that it's happened multiple times. Mind you this is at the end of 25 miles commute and 1 hour drive. To answer some questions, I use ECO mode, I do have the A/C in circulate mode, no one in the back seat, nothing in the cargo area. I also checked and the defroster is not on. I tried pressing the EV mode a few times but I don't want to share results b/c i wasn't really paying attention to all the facts so I don't want to mislead anyone. I will check the other items mentioned and provide an update. Thanks again.
Quick question. When you're coasting is it down hill? Is it a long slope or long amount of coasting? If so, on the hybrid indicator where it shows the shape of the car, are the red arrows moving about when the engine runs whilst coasting? What is the state of HV battery charge when the engine runs? Could it be the HV battery is full and running the engine without petrol (if it does this the red arrows don't show despite it being obvious the engine is running) to burn off some of the excess charge?
What you experience is an idling check ceremony. There's nothing wrong with your car. I have this same thing goin on after my commute on the highway. If I come to a full stop, there's no Idling ceremony. If I happen to catch a green light directly from the highway I have an idling ICE. Only way to stop it is at speeds below 35 km/h from a glide or a fullstop. After that there's no more idling. Only question that remains is why it needs an idling check ceremony... Hope this helps.
GrumpyCabbie, no the four times I've noticed it this week was on level surfaces. However, to your point, I believe all 4 times the HV battery was two bars from top (normal SOC position when full). It happens too quick for me to flip the indicator and look at the red arrows but I'll try next time it happens. Ulgymuddah, yes similar to an idle check. Some have suggested that the car for some reason is reverting back to a stage 3 or stage 2 warmup but after 25 miles I don't see why. Others have pointed out that HV battery is not getting enough air or that the thermostat is faulty.