Prius Engine Idling at at Stop

it doesn't sound like the 12v, but it's worth testing. could be some basic maintenance item, i would have the codes checked. maybe pcv, throttle body/maf, erg, plugs and etc. are you up to date?
it's also possible your hybrid battery has a weak cell.

 

Even in freezing temperatures the ICE isn't going to idle for 30 seconds to charge the traction battery or just to warm up. All the 12v battery does is boot up the computers. The 12v may be weak but that isn't the problem

 

Oh, I don't know. It could be a weak 12V. I know when a Gen II does this it is a good item to check. The Gen III probably does the same thing. Plenty of threads in the II and III forums about that little 12V battery. If it is the original (like mine) it might be time to replace it. I use a conditioner (Optimate 5) once a month to keep mine in line. So far, so good.

 

Around PC, the 12v can be the source of pretty much any issue a Prius can have, and certainly check the voltage with a multi-meter. But considering a 2011 at 127k miles, you spend enough time in READY mode that your 12v is well looked after by the HV system. Unless you're bad about leaving dome lights on or using accessory mode, I'd guess the 12v is fine.

To bump Bisco a bit, if you have no check engine light, I'd start with a careful cleaning of the MAF and throttle plate/body. Mop the puddle of oil from the floor of the TB as best you can, too. If these have never been cleaned, it's likely the problem, imo. Good luck!

 

I have issues with engine not shutting off at stops if brakes were applied too hard. I think ECU assumes that you are in accident avoidance mode and keeps ICE running in case if you need extra power to get out of the way.

 

My 2013 has done this since new and it primarily shows up when it is cold and I am almost done with my 40 mile commute. One dealer said there was "nothing wrong", which sounds like "no codes to read", and another wanted to charge me $100 if it wasn't a problem to repair under warranty. So knowing there wasn't a code, I figured they wouldn't find anything.

 

If the heater is on the ICE may stay on when the car stops. The ICE supplies the heat. Try turning the heater off before you come to a stop. I think the ICE then would shut off. I've long noticed this behavior. If I'm trying to squeeze higher mpg's out of the car I'll turn the heat off for this reason.

 

After 100 miles at 66F coolant should be well over 150F. If the ICE is running because the heater is on then something's wrong.

Agreed that turning on the climate control or defroster before temp is 150F will cause the engine to run as it tries to warm everything up.