07 P2195 DTC, A/F sensor or something else?

Hi all,

I'm a broke college student that is trying to figure out if the issue I have is something I can diagnose and fix myself to save money. I have a 2007 Prius with ~212k miles that recently had the check engine light come on while driving in some heavy rain. Everything felt normal and I was almost to my destination so I waited to park and check with the OBD-II scanner. The scanner pulled up P2195 (Sensor 1 Bank 1 Signal Stuck Lean) and P0138 (Sensor 2 Bank 1 High Voltage). A quick search suggested it wasn't a critical issue so I tried clearing the codes, but they popped up again on the way home.

I'm not very familiar with cars but some research leads me to believe that the two most likely culprits are the upstream A/F sensor needing to be replaced or a vacuum hose leak causing the codes to go off. It seems less likely to be the downstream O2 sensor, as that's probably being triggered by the root cause of the issue. Here's some notes about the car/recent maintenance:

• The car doesn't feel like it's idling or running rough at all, and seems to have normal power while driving as far as I can tell.
• There's no smoking or burning that I notice.
• The fuel mileage has dropped from about 44mpg to about 38mpg, which is a red flag to me.
• I replaced the inverter coolant pump myself about 1000 miles ago, I don't recall there being much in that area of the car that was related but I wanted to note it here just in case I might have bumped something.
  
• I had the oil changed at a quick service place a couple hundred miles ago.
• I replaced the engine and cabin air filters myself immediately after the oil change using the correct part numbers for the car. I've double checked the engine air filter seal and orientation since and everything looks fine, reseated it and tried clearing codes but they come back still.
• I've checked the hoses that I can access under the hood without removing anything and everything looks good, all of them are clamped in place and there's no rotting or burned hoses that I can see.

Does anyone have any insight as how to further diagnose this, especially with regards to checking for air leaks? The engine bay is thankfully pretty clean, so I can see things pretty clearly. I have a bluetooth OBD-II scanner and the Dr. Prius app, if either of those are helpful. It looks like replacing the A/F sensor is kind of a pain in the butt and requires a special crows foot socket plus working in a tight space, so I'm hoping it's just a hose or something. I have a mechanic I can take it to if it ends up being a wild goose chase or service that's beyond my skill/tooling, but I appreciate any input you all have!