Low mileage after a lot of idling with ac on all the time

Any lights showing on the dash, and MFD?
You didn't state the miles on her. Probably you need to do some tune-up on the engine, such as spark plugs replacements and all that.
What kind of tires and tire pressures do you maintain on your car? All that have impacts on your MPG.
Finally, might be that your HV battery is already showing some signs of weakness. Definitely, in hot climates, for instance in Africa and the Middle East, batteries deteriorate more.



Dxta

 

If there aren't codes , and all that, your car might be OK. Could just be suffering from the impacts hit temperatures is having on the battery pack.

 

First, be sure that you are actually looking at a mileage number that is reset for each trip......or manually reset it yourself.

IF.....you are looking at a running average, then a "bad" spell will drag down the average, theoretically forever.

 

Check that the vent in the back seat isn't blocked. If it's not, it could just be that the temperature is extremely hot. Even without running the AC and rolling windows down, the car still doesn't like it when it's above ~90 from my experience.

 

12-14 km/Liter is definitely too low for long trips, unless you are driving extremely fast. And even then I can't imagine a C giving mileage that low.
You said the hybrid battery fan vent was clean and unblocked, but I would check the fan itself. Two clips hold the panel on. Here's a video that shows how to do it, with a pretty dramatic shot of how dirty the fan's mesh filter inside can get.

Also, I would up the tire pressure to at least 36. Many Prius owners use 40 and higher, but I find the the small mileage gain not worth the loss of ride comfort. I average 50-60 mpg at 36 psi. 32 is very low.

 

None of this tells us why you think this is wrong... just that one of your trips was good, but another was not. I have an OBD attached to my phone, using Torque and the readings are pretty close.
I have never personally seen a tank that was off more than 1 mpg from my hand calculation. I attribute that to the car being more knowledgeable about how much gas it is using in the injectors, whereas I have no real idea about how much I am really putting into the tank, since gas pumps aren't super accurate.
I've had to sleep in my car overnight due to a blizzard while driving thru Kentucky on my way to Minnesota. Kept the car nice and warm ~80 degrees with the seat warmers on for 9 hours. That was the first time I found out that the car goes below zero F. That was my worst tank, but still got 35.77. Was scary seeing other drivers turning their cars off to save gas. I only used 2 blips on the gas gauge for the 9 hours.

 

It is really very simple.
When you use fuel with idling with A/C on AND you don't move much, the average goes down.
But a lot of people don't seem to understand the simple math of an average.
Miles driven divided by fuel used. If the fuel used goes UP and the miles driven doesn't, your average suffers.

 

The internal thermostat that shows ambient air temperature. It was -5 outside and the temp reading was -5.