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Discussion Starter · #1 ·
Reading the Chilton's Manual, it states that the Upstream O2 sensors are responsible for air/fuel mixture of 14.7/1.

It also states that the downstream O2 sensors have nothing to do with air/fuel mixture, but that they are used for other calculations.

I replaced the upstream sensors today, but should I replace the others as well? Car has 125k on it.
I cleaned the air filter, throttle body, MAF and replaced the upstream sensors, in an effort to get better gas mileage.
Car was getting ~20mpg HIGHWAY regardless of whether I drove like a grandma at 65-70mph, or like a hooligan, at 75-85mph, accelerating quickly.
 

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Downstream only need to be replaced if there's a fault with one (tosses a code)
 

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Downstream do pretty much nothing except to tell if the cats are working or not.

I have a loose wire on the downstream O2 on my Prizm now. Since it is something with the wiring, it's intermittent, so I just clear the code, do a drive cycle to make sure the light doesn't go right back on, then get the sticker! Light usually comes on a day or 2 later, but at that point, it doesn't matter for another year. Gotta love OBD2!

So yes, don't even worry about them unless they actually start throwing a code.
 

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Discussion Starter · #4 ·
Thanks!
After changing the 12 year old O2 sensors, and cleaning the MAF sensor, it starts right up! Must less time with the key at the "start" position.
 

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OPD not reporting - no sticker

Downstream only need to be replaced if there's a fault with one (tosses a code)
I have a 2001 Taurus which won't pass inspection because the catalyst monitor and the evap monitor are "Not Ready". Everyone tells me to drive it around, so I have..like 400 miles on the highway. NO CHANGE. ANyone know what's wrong? I had no electrical disconnect previous to inspection, and no engine light on. I'm at the stage where I have to belive that SOMETHING needs to be replaced.
 

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@kdmcgann

Not sure how to help you with the catalyst monitor but the evap monitor needs to have more than 1/4 tank but less than 3/4 tank to run.

I don't know why the catalyst monitor would stay at not ready, unless you go on drives that are so short the cats and the 02s don't get to heat up, but if that was the case, there'd probably be a lot more monitors that are still at "Not Ready".

I'd figure that if there was a problem causing that then some kind of code would have been thrown by now. :confused:
 

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Discussion Starter · #7 ·
I had the same issues. Frustrated with the 100 miles and ~1/2 tank I dumped from just driving on the highway, I ended up driving to Georgia from North Carolina and back to get the "drive cycle" to finally take, including some time in between on secondary roads.. The drive cycle apparently goes through after you have been driving 35-45mph for some time. Highway driving did nothing for me. However if that is with those "certain" aspects of the drive cycle, then I'm not sure. Best of luck!
 

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EVAP also wont run when ambient temps are below around 40 deg F. So, if you live in the great frozen north, EVAP test wont run all winter.

There is a specific Ford drive cycle that will (supposedly) allow all the IM monitors to complete. It is probably posted around here somewhere, or do a google search.
 
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