Significant Drop in Gas Mileage

Prime head gasket failure mileage. You have P0301 (misfire on cylinder one), which pops up almost without fail with 3rd gen head gasket failure (usually starts at the wall between cylinder one and two).

I'd assume the EGR cleaning was done recently, which is very late-in-the-day, should be done at least by 100K. You want to thoroughly clean all the EGR components, which is everything between the exhaust manifold and the engine intake ports, including the intake manifold. If someone else is doing your EGR cleaning, and they're not experienced at it, they've likely missed cleaning something, either the EGR cooler (connects to the exhaust manifold, is a bear to clean) or the intake manifold (didn't know it needed cleaning).

But deal with the head gasket first, try the aforementioned leak down test and boroscope inspection.

 

Hello.
My wife bought her 2018 prius three new in November of 2018. The only thing we know about the car is the lowest model besides the color that she really like is the white and its really good in gas. since she is just working at home and hardly using it right now which is presently 5,100 miles, we took the car for a trip. I'm very surprise that this car saves way more gas than my well-kept 2007. I was close to 500 miles when we filled the tank. Is there anything I need to watch with this car as far as maintaining it. Thanks,

 

just follow the schedule, and keep an eye on the coolant level, in case you get the dreaded exhaust leak

 

Thanks very much for that link, Mendel.

I've been chasing what now I see as unobtainable, so now I'll just accept the 19 or so km/litre and totally relax about it.

gratitude from New Zealand

Very few are getting 4.5, judging from Fuelly: