I have a 2003 Ford Explorer XLT with 126000 miles on it?

I have a 2003 Ford Explorer XLT with 126000 miles on it?
Added (1). Last week the service engine soon light came on so I had the code read (P0035) so I took it to a mechanic and he said it was the o2 sensor, which he replaced. I drove it for about 30 miles with no problems and now the service engine soon light is back on. I know it needs a tune up, could the light coming back on be related to the need for a tune-up, or something else. The sensor replaced was in bank 1.

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