The mpg on my 99 4.6 seems to have gone down so I decided to freshen the O2 sensors. At 105,000 I changed the pre-cats only. Now I seem to be getting a little less mileage. Have I thrown the computer for a loop? Do I need to change the back two as well? What about clearing out the computer so it can start from scratch?

Also what about other ideas to get my mileage back up.

 

It is recommended that you reset the computer whenever changing sensors in a vehicle. Just disconnect the battery for a while and it should do it..The rear O2s are there to monitor that your cats are there and working..I don't think they would effect mileage.

 

the 02s absolutley do affect mileage, the monitor lean/rich conditions, and adjust your A/F ratio accordingly.

Just unplug battery, turn headlights on, wait 5 min, turn headlights off, plug battery back in. The headlights are just to help drain any left over power after battery is disconnected. or you can wait 30 min....
-Patrick

 

Not the rear ones. They ONLY monitor catalyst efficiency.