I have a 2000 Discovery series II with 123,000 miles. I changed the front 02 sensors at 90,000 miles because the engine light came on and there was a code. Should I change the rear 02 sensors just for maintanance and hopefully a little more fuel economy? The engine runs perfect with no lights. I Just installed bosh +4's and magnacore 8mm., That seemed to add a little lowend power and it runs smoother.

Thank you,
Willie

 

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They should be changed in sets. All four at a time. They start going out around 80K...

 

I agree, need to change out all four at the same time. By doing this you may still gain a little extra kick from the old engine.