My car at a full tank of gas usually have an estimate milage per the fuel indicator of 425 miles. Recently, when I filled up my tank I noticed it went down to 380 miles. Then weeks later my check engine light came on: O2 sensor. So we reset the check engine light, purchased fluid injector cleaner and added premium gas. The check engine light is no longer coming on, but after the use of the FIC running the full tank of gas, I am still getting bad readings off the digital fuel indicator. Prior, my indicator light alerts for low fuel at 50 miles. Today I noticed it came on at 40 miles. 1. Do you think the two issues correlates? I think I do, maybe I have a clogged? But where? 2. Should I do a second round of FIC? 3. Where do I start?
Considering you are getting a CEL for an O2 sensor and your estimated fuel mileage is down, would it make sense the O2 sensor(s) might be the problem? If the front O2 sensor(s) is bad, this is exactly what I'd expect to be happening. Before you assume there's an issue with something else, I'd suggest you consider putting new O2 sensors in it and reset the ECU so you're back to a stock fuel map.
First thing you should do is to check what is the diagnostic code you getting , you can do it by buying OBD2 tester ELM327 only around 15$ If you get p420 p430 you have problem with your catalytic converter.