2013 prius c no codes but reduced mpg's

that sounds like a software issue. are you calculating mpg at the pump, or watching the screen?

 

...possibly eco mode, not ev mode?

 

you should be able to tell if the engine is running by sound?

 

not everyone is sensitive to sound.

The c does have the little "car with EV knockout" lamp on the throttle scale display in at least one of the standard dash displays. That will always be honest about whether the engine is running or not.

I never look at that one myself, we leave our car set on the so-called "simple" display that just gives fuel level, time and temp.

We deal with traffic, we let the computer deal with fuel economy. It works for us.

 

it might be worth calculating at the pump and keeping a record

 

Thanks for the praise Elmo, but that was Leadfoot who mentioned the low refrigerant as a possible cause.

 

Might not even be a low refrigerant situation. There is such a thing as a day hot enough to completely tank your mpg through cooling demand, even on a healthy system. You may have just found a string of those days that you weren't used to.

Either way it sounds like you've discovered where the energy is going. Now to work out whether it was appropriate for your travel conditions, or if the system has gone low.

Given a 10+ year old car, I'd probably spend the $80-90 for a "system checkout" at an independent automotive AC shop.

 

Ah, in that case you now know that the refrigerant in there is the wrong amount. These are sensitive systems, and having the wrong amount of refrigerant (high or low) by just a few grams can really knock it out of kilter.

If you've blindly added some, the right move would be to have all of it removed/recycled and then pay somebody to refill it with precisely the right amount.