2008 Prius MPG Issues

Welcome! The cold has an impact, but not that drastic to where you would see 25-30 mpg. (Don't think you should ever see that unless you're driving like 90 mph). Is your driving more then 5 or 10 minutes at a time? The first five minutes on the display will show that low from the warm up cycles, but I am guessing your drives are longer. I have read on other forums that some cheap aftermarket catalytic converters can cause the drop, and seeing as it times perfectly with your situation that would be a likely cause. You seemed to be thorough with your troubleshooting, so I will let someone else give their input.

 

I did an advanced search with "catalytic converter mpg" The first one had the same drop in mpg upon replacing the cat with an aftermarket. Said it was fixed by cleaning the mass air flow sensor. Maybe try that out. maf Cleaner is like 6 or 7 dollars and its an easy part to get to.

 

Have the 12v checked under load at parts store. They’ll do it for free. Takes two minutes.

What brand and model of catalytic converter did you get? If the guy tells you he doesn’t know, that’s a red flag.

Get a OBD Bluetooth code reader and see if any codes have been triggered. short trips are a two prong problem in colder weather, they don’t charge a cold 12v battery and the gas engine or ICE runs longer.

You can also use the free Dr Prius app to check HV battery.

 

You need a better aftermarket catalytic converter, and that grinding noise (it's probably a rattle) is your aftermarket catalytic converter vibrating against your new shield.

Cheap catalytic converters aren't always cheap in the long run (poor gas mileage). I bought a fairly pricey one:

And my mileage is still 45-50 mpg.

As far as the rattle goes, I would get new rubber exhaust hangers, which should raise your catalytic converter away from your shield.

Rubber Exhaust Hangers