2001 Saturn L300 service engine lights

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I have changed 3 o2 sensors in my car. i keep getting a code 50 when i get a test done ive change the 3rd sensor in front of the manifold, drove the car around and the light still stays on.

You have to erase the stored trouble codes after making the appropriate repair. If the codes are erased and same code(s) set again let me know..

how do you erase the codes. i was told to drive the car and let the computer reset itself. but its not working.

No, that's not correct. You have to use a diagnostic scan tool to reset the codes OR you can disconnect battery for a few minutes and reconnect however this is not the best way because it resets more than just the computer, it resets radio presets , clock etc.. Your local auto part store should clear the codes for you free of charge if you ask.