[JKoob]

Had a bad O2 sensor that killed the catalytic concerter on my 2000 Forester. Replaced both of them but even with the bad sensor I was getting above 25MPG. Now with the new sensor I'm getting just above 20MPG.

It's only been a week since I replaced the sensor and the cat. Does it take a while for the ECU to readjust itself? I did reset it and the CEL has not come back on, and the car does seem to performing much better already. This seems to be the only hiccup.

 

[Real Lyte]

Give it a few driving sessions for the ECU to "relearn".

 

[JKoob]

I figured that...

but some 300+ plus miles later it doesn't seem to be getting better.

 

[Real Lyte]

but some 300+ plus miles later it doesn't seem to be getting better.

Ah, in that case it is possible that the new cat needs to be broken in or some else is not working to par, have you done any recent tune ups (plugs/wires/filters)? You could also try resetting the ECU by disconnecting battery for 20 minutes and hitting the brake pedal a few times.

The reason for lower mileage might even be because before the sensor was causing it to run leaner than it should at times, using less fuel.

 

[JKoob]

Thanks for the info!

 

[SPDRPIG]

Not bad. My 2000 S with 105k miles NEVER gets better than 19. Now with the new 215/16 AT's it's about 17 MPG...maybe.

 

[Real Lyte]

Also don't forget if you noticed the mpg change when cold weather started it can also be due to winter gas.