Does the after cat o2 sensor affect fuel/mpg.

Most threads and other sites say NO. Toyota says YES.


URD says "Toyota started using a wideband type O2 sensor (air/fuel ratio) in the front position for better engine control. These wideband type sensors need to be recalibrated from time to time by the ECU. The ECU uses the rear sensors for this recalibration process"


Who is right and why?

 

I'd guess that URD is right, Gadget is pretty good at doing his homework.

 

Rear o2 sensor in an ODBii system is only to tell the ECU if the cat is working correctly. The front O2 sensor is the one that is used for a/f mixture. If the rear one was used to control the A/F ratio it would see way too much O2 because the cat converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen.

 

This is not true; the rise in oxygen after a 3-way catalytic converter is not very large due to the reduction of NOx, and is well within the range of a narrow band (HEGO) sensor.

In the industrial engine world, some intelligent air/fuel ratio control designers are using a post-catalyst oxygen sensor to account for catalyst performance degradation due to aging. The pre-catalyst O2 sensor is used for air/fuel ratio control, but the post-catalyst sensor can bias the pre-catalyst O2 target by a small amount based on changes it sees after the catalyst over time.

I suppose there is a possibility that this is also true for some automobiles, but your average vehicle sees far less use than many industrial engines in its lifetime so I don't see this being as beneficial. I have always been under the understanding that the post-catalyst O2 sensor was required for OBDII vehicles to verify that the catalytic converter is installed and (somewhat) functioning.

-Dan

 

Well... I'll find out one way or another, I'm getting horrible mileage, turns out my post-cat o2 sensor is not working. So I'll replace it and it will hopefully fix my mileage (not likely) or maybe it will tell me my cat isn't functioning properly (more likely) and that is causing my bad mileage.

 

are you getting a cat efficiency CEL?

 

No, I'm getting a bank 1 sensor 2 malfunction CEL. AND the ecu depends on the o2 sensor and it's getting no signal at all from the rear o2 sensor so it's basically not performing the cat diagnostic. (Which is confirmed by my OBDii scanner that the cat info is not being reported)

So I suspect/hoping once I replace the o2 sensor and the ecu sees the readings it will probably trigger a cat code. Otherwise I'm back to square one with no idea why I'm getting 10 mpg on a 3.4L

 

ewww, no power loss or?

 

No, seems to have plenty of power, and drives real nice, shifts smoothly. Although at times it seems like something is holding it back a little. But it's not the brakes I had those checked out.

Although it did have a misfire when I first bought it a few months ago, and I have no idea how long the previous owners drove it that way. But it just needed a new coil pack and that fixed it. But maybe all that unburnt fuel from the misfire ruined the cat.

EDIT: And I took it to Toyota... and paid them $100 to tell me the MPG is related to the o2 sensor after the cat. Which I don't think is true but they said for sure it is true so who knows. But thats the reason for this thread lol.

 

very possible. I ran about 6 months on a CEL in an old import racer and killed the cat. Stupid me. that was like 10 years ago. lol.

 

will a bad cat cause a false positive for a rear o2 sensor malfunction?

 

Well I just replaced my rear o2 sensor finally. So in a few days we'll see if I get the same CEL, or I get a different CEL for the CAT. But as of now, with the new rear o2 sensor, my mileage is still horrible.

 

Did your gas mileage improve after Rear O2 change?

I have the same code PO141. Bank 1 Sensor 2. The cat is only a few years old. I am going to replace the sensor soon and hopefully mpg's increase. Recently I just replaced fuel injector 4, code PO304, and had all injectors cleaned, and replaced all top end gaskets, manifold and valve covers. Running better than ever at 304,000 miles. Now this stupid code to fix up. Rock Auto has the OME denso's for only $65 front, $67 rear. Even though I did them about 2 years/50 k ago. I may just replace both again, or should I wait for the front sensor to go?

 

so im kinda getting the same issue right now myself. i keep getting the p0420 code. so i hooked it up to a scanner the other day the scanner read that the front o2 signal was not moveing much but the rear one was jumping all over the place. well i know this should be the opposite so i replace the front o2 sensor with a new denso. well after running for about 2 hours i tested it again and was getting the same signal as before. so i know that the sensors are working but im not sure why they are reading this way. i think its going to be the cats but i guess i wont know till i replace them. anyone have any info on what these o2's should be reading?

 

did you anyone ever conclude a definitive answer to this?

 

http://www.autoshop101.com/forms/h37.pdf

 

very interesting. thx. that improved my understanding of exhaust by 95%