Poor mileage after O2 sensor replacement

[mtthmpsn]

I haven't been on here in a while as I've had good luck lately with my '06 3.0i (218k miles). That said, service engine light came on and called for bank 2 pre-cat 02 sensor replacement. Extremely easy replacement (replaced both pre-cat sensors), I reset sensor adaptation with INPA, and SES has stayed off.

Problem is, it seems like I'm getting about 2mpg less than previously on the last 2 tanks of gas (same driving habits/locations). Is this due to the adaptation reset and should I expect it to improve? Or due to the sensor replacement itself - I would think 02 sensor replacement should IMPROVE mileage?

Any insight would be appreciated.

[andrewwynn]

Originally Posted by mtthmpsn

I haven't been on here in a while as I've had good luck lately with my '06 3.0i (218k miles). That said, service engine light came on and called for bank 2 pre-cat 02 sensor replacement. Extremely easy replacement (replaced both pre-cat sensors), I reset sensor adaptation with INPA, and SES has stayed off.

Problem is, it seems like I'm getting about 2mpg less than previously on the last 2 tanks of gas (same driving habits/locations). Is this due to the adaptation reset and should I expect it to improve? Or due to the sensor replacement itself - I would think 02 sensor replacement should IMPROVE mileage?

Any insight would be appreciated.

This is a very normal happening with winter fuel formations and could be the entire reason. I have a friend who logs all of his fuel fills and every year around November his mpg drops about 5 mpg until spring.

[mtthmpsn]

Originally Posted by andrewwynn

This is a very normal happening with winter fuel formations and could be the entire reason. I have a friend who logs all of his fuel fills and every year around November his mpg drops about 5 mpg until spring.

Interesting. I didn't even think of that, but makes perfect sense. I'll keep an eye on the mileage as the seasons change. Makes me a feel a little better that it's probably not due to the sensor change. Thanks.

[zener]

And what was the exact code you got?

[mtthmpsn]

Originally Posted by zener

And what was the exact code you got?

Sorry, I don't recall the code exactly. I don't always trust my OBD II reader code, so use INPA and my laptop to read the errors and it was pretty clear it was the pre-cat bank 2 O2 sensor out of range (after German-to-English translation in Google). Since I haven't seen the SES light since the fix, I think INPA was right - just coincidental I guess that my mpg seemed to drop after the repair.

[andrewwynn]

When I replaced my front O₂,7b first drive had amazing mpg then it dropped back down and was no different or worse.

Did you reset your fuel adaptations after the repair? If not that could easily be the reason.

Tye car will figure out over time what long term fuel trim has to be and if it's off mpg will suffer.

Ambient temperature will affect mpg for a few reasons so I think you may have to ride out a tank of bad mpg and see what happens longer term.

Oh: check your coolant temp real-time. Make sure you are getting to closed loop: I lost a lot of $ paying for extra gas when my thermostat died and I was running rich all the time.

[mtthmpsn]

Originally Posted by andrewwynn

When I replaced my front O₂,7b first drive had amazing mpg then it dropped back down and was no different or worse.

Did you reset your fuel adaptations after the repair? If not that could easily be the reason.

Tye car will figure out over time what long term fuel trim has to be and if it's off mpg will suffer.

Ambient temperature will affect mpg for a few reasons so I think you may have to ride out a tank of bad mpg and see what happens longer term.

Oh: check your coolant temp real-time. Make sure you are getting to closed loop: I lost a lot of $ paying for extra gas when my thermostat died and I was running rich all the time.

Glad to hear other's experience on this. If fuel adaptation reset is different than the lambda sensor reset, I'll look into that (I only reset the lambda sensor adaptation). And good advice on coolant temp - I replaced my thermostat less than 2 years ago, but I'll keep an eye on it.

[zener]

Originally Posted by mtthmpsn

Sorry, I don't recall the code exactly. I don't always trust my OBD II reader code, so use INPA and my laptop to read the errors and it was pretty clear it was the pre-cat bank 2 O2 sensor out of range (after German-to-English translation in Google). Since I haven't seen the SES light since the fix, I think INPA was right - just coincidental I guess that my mpg seemed to drop after the repair.

Being out of range usually doesn't mean lambda is the culprit. You could have problem elsewhere. Maf related as I had (aftermarket maf did not work properly)... And many more. With new lambdas it might take sometime to problem occur again.

I also had some vaccuum leaks.

[mtthmpsn]

Originally Posted by zener

Being out of range usually doesn't mean lambda is the culprit. You could have problem elsewhere. Maf related as I had (aftermarket maf did not work properly)... And many more. With new lambdas it might take sometime to problem occur again.

Understood. Obviously, fingers-crossed that's not the case here, but will repost on this thread if the sensor error returns or if mpg doesn't improve in the spring/summer.