P0171 followed by P0136/0137 Diagnosis and repair

2011 Prius Currently at 279k miles

Check engine light came on. It was P0171
Plugged in my scan tool. stft was normal but LTFT was 30% (should be less than 10)

about 65,000 miles ago. I did the head gasket with all new seals, new spark plugs, new injectors, new coils, new PCV valve, cleaned the EGR, pretty much everything you would want to do so I was hesitant to start checking for leaks under the hood.

symptoms that I noticed:

-the check engine light would come on on the highway after being reset.

-when I was driving through the mountains. The car seemed to be a little flat going up the hills (The Prius is of like this in generallol so I didn’t think much of it until the check engine light came on a week or two later)

-my gas mileage had dropped considerably since when I did the head gasket

based on the mileage, fuel trim data, the lack of performance uphill, and a YouTube video that mentioned the P0171 light coming on on the highway being a symptom of a bad fuel pump, I decided to replace that first.

once I replaced the fuel pump. My fuel trim came back into normal range and I was no longer getting a P0171 light, but instead, I had two codes now for a P0136/P0137

again. I looked at my live data and I noticed that the voltage for the bank one sensor 2 O2 sensor was not getting a consistent or expected voltage

not sure if this sensor went bad because I was running lean for a few thousand miles before I got around to doing the fuel pump, but I replaced the downstream O2 sensor, drove 100 miles with no lights and that cleared the permanent codes

in addition I just broke my gas mileage record on the first tank since replacing the sensor, 49.7 miles per gallon over 452 miles and that included another trip to the mountains of New Hampshire.

hopefully this can help somebody else!