Rear O2 Sensor faulty?

[kruegerk]

I've been trying to diagnosis both a significant reduction in fuel mileage as well as a rich exhaust smell. I hooked up the vehicle to my scan tool and have monitoring the O2 sensors. The two forward sensors seem to be oscillating between the high and low voltage that is called for. The rear O2 sensor seems to be holding a steady and very low voltage. I've attached an image while at idle and a slight amount of throttle. Should the rear O2 sensor oscillate as well or does it hold steady?

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[07gtman]

hope this helps i know its a little long but easier than tryng to explain it

 

[lincolnshibuya]

any codes at all? if you have bad downstream sensor it should show a code (or pending) the downstream doesn't also affect your fuel economy (it's the upstream ones that's being used by the ECU)

 

[kruegerk]

Thanks for the video and the feedback. I'll spend more time reading the scan tool today. I have read where the downstream sensor doesn't effect mileage but I've also read where it can. It does make sense that if the downstream sensor is reading lean that it would communicate to the computer to add fuel to richen the mixture up though. It does have some voltage so it may not be bad enough to throw a code which I don't have yet. I'll keep testing..

 

[lincolnshibuya]

downstream is only for checking if the cats is working, it doesn't have to do anything with your lean or rich condition.
the upstream does all the work sending feedback to the ECU.

 

[kruegerk]

Did some more scan tool work and it looks like O2 sensors are reading on the lean side. Bank 2 looks to be more lean than the other. I'm guessing at 120,000 miles these sensors are starting to fail but haven't failed enough to set a code

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[07gtman]

did you check maf sensor could be dirty

 

[kruegerk]

did you check maf sensor could be dirty

That was one of the first things I cleaned.

 

[07gtman]

idle air control valve would be something to look at it may need to be cleaned or replaced