98 I30 here.

The SES light came on and it was the O2 sensor code. Bank 1 sensor 1. While I was doing the search on it in the deals, the SES light went off now. I am guessing the light might come on, isn't it true ?!? I did notice that the car has more power now after the O2 sensor gave up

Do I need to change the O2 sensor (thinking of changing both the front ones as they have worked for 114k miles already!) even if the code/SES doesn't come back ?

I just went on a roadtrip to Indy each way 220 miles and the car gave reasonable highway mileage of abt 25 mpg But during last week, it gave 15 ~ 16 mpg in city driving

 

When I get a CEL I always clear the code first and wait and see if it comes back. If it comes back then I replace. Never the first time.

 

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thx mike and i30ds ... i was just hopin that the CEL comes back on so that i can put the new o2 sensors in to get better mileage i know i know there are 101 things that affect mileage ... but are the 114k miles too much for the o2 sensors to work properly or am i lucky here ?

anyways, i didn't clear the codes but it just went away. i'll wait for the code to come back. meanwhile, i'll check for ghost codes.