2017 Prius Check Engine Light

What was the actual trouble codes that the mechanic found. Without the trouble codes, everyone reading this thread can only guess what the problem is. The probability of guessing correctly is very low.

 

Need error code(s).......
Modern ECU, fuel injected cars DO NOT need warm-up before taking-off. You should take it "easy" on the engine until it warms up, but does not require 10 minute warm-up. If your battery level (SOC) is high, putting the car into Ready mode won't even fire the engine anyways - so how is it warming-up?????
The 2016-2018 Prius are prone to exhaust gas heat exchanger leaks. If your coolant level is low that can cause temperature issues, but I'm just guessing without the error codes.
Something is off or your misunderstanding the mechanic or the mechanic doesn't know what they are doing. As I stated in the previous paragraph, putting the car into ready mode doesn't guarantee an engine start.

 

Yeah, this is not a thing with any modern car.
It's nice for the driver if it's 15°F.

Keep an eye on the coolant level and swing by an auto parts store that will read the codes for you.

 

Your mechanic didn't fix the car; because your problem came back!!!! Find someone that specializes in Prius, that understands these cars. More than likely an intermittent sensor issue, that your sister's driving style is somehow triggering. Just because your driving style isn't triggering it - doesn't mean there isn't a problem. That's why we need the error codes to figure it out!!!

It's NOT your sister's fault; because I routinely place my car into ready mode and take-off out of my driveway - WITHOUT issue; since day ONE from the dealership....... I manually switch the car over to HV mode when I hit the highway 8 minutes down the road. That's when my engine kicks on from a dead cold start; moving 65+mph....... This is a car; not Cinderella's glass carriage. If anything, you should be thanking her for alerting you that there is a problem with the car. Early diagnosis and fixes are usually a LOT cheaper than letting a problem fester and getting more expensive to fix.