Engine Symptoms: Rough Idle, Poor Gas Mileage/ Acceleration?

Yes, those Check Codes are pointing to exactly the reason for your problems.

P0171 - System Too Lean (Bank 1)

P0174 - System Too Lean (Bank 2)

P0441 - Evaporative Emission Control System Incorrect Purge Flow

The first two codes are obviously related to each other, and it's not surprising that they both came on at the same time. "System too lean" means that the air-to-fuel ratio is too high, too much air getting in, not enough fuel. The optimum balance of air and fuel is 14.7:1 ratio. In your car it might be higher, like 16:1 or 20:1 or whatever. So you need to get the engine's ratio back in balance. Changing the O2 sensor might have been premature. What they usually suggest is to clean the MAF (Mass Air Flow) sensor. Other problems that might cause this could be cracked hoses and PCV lines, or a dirty fuel filter. Next time, I'd suggest not relying on Midas, for this problem, I'd suggest going to a Mitsubishi dealership to get it fixed, just for a reliable resolution to the problem. Midas is good for oil changes and simple stuff like that.

As for the 3rd code P0441, that may be completely unrelated to the first two. Usually this code comes on if you forget to close your gas cap, or you don't close it tightly enough, so it may be just a one time thing where you maybe forgot to close the gas cap. It's a warning that fuel is evaporating away. Sometimes the problem is more than just a loose gas cap, and the mechanic will need to investigate further, but 90% of the time it's just the gas cap.

Rough Idle Symptoms