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Old 07-21-2012, 10:32 AM   #1
Joboo
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Question about post-cat O2 sensor

I've spent the last year () trying to diagnose a problem with my wife's 2003 Ford Focus SE (Zetec 16v). It is currently experiencing the dreaded, mysterious rough idle and stalling. It's bad enough that it basically stalls in anything but park or neutral. I have gone down the usual list (coilpack, IAC, EGR, MAF, PCV hose/valve). Every time I think I've got it... .

So currently I am attempting to diagnose the O2 sensors. I don't have an oscilloscope but I do have a real-time sensor monitor that can at least give me a basic picture of what's going on. My understanding of how things should be working: sensor 1x1 (pre-cat) fluctuates between 0.1v-0.8v as it responds to the amount of oxygen in the exhaust gasses. Sensor 1x2 (post-cat) should offer a stable reading if the cat has done it's job. If not, it will set a code indicating a bad cat, but either way it is just a passive sensor and the ECU does not use sensor 1x2 to adjust airflow or fuel trims in any way (i.e. even if it were messed up in someway, the car should run fine).

Here's what's actually happening: sensor 1x2 is giving erratic readings. Not constant fluctuations like 1x1, but spikes here and there. Like I said, my assumption would be that this SHOULD set a code (no codes at all, pending or set) but that it shouldn't effect idle anyways.

So I clearly have some incorrect understanding of how the system should work, and possibly some faulty equipment as well.

Any thoughts or help would be much appreciated...

JB


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Old 07-21-2012, 12:34 PM   #2
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That is strange your not getting a code from the sensor not functioning properly. My post cat sensor wasn't functioning correctly and mine threw a code. I had a rough idle. I went ahead and changed both sensors. Takes less then 30 minutes. Car runs and idles baby smooth now, and has much better throttle response.

My assumption would be that it sets a code too?? So I'm kind of confused. I would go ahead and just throw a new sensor in there considering it takes a few minutes, if your funds allow.. How many miles does the car have if you don't mind me asking?
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Old 07-21-2012, 12:52 PM   #3
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I think you're chasing a red herring with the rear o2 sensor. It's there just to check cat efficiency; it's not part of the A/F ratio adjustment feedback loop.
In my case, an erratic idle was a vacuum leak in an odd position: Where the EVAP purge line - the large one - enters the intake manifold, there is a rubber O-ring hidden within the "twist to tighten" fitting. Wiggle the fitting and listen for a hiss or change in idle quality.
Regardless if it's that or not, re-check all your vacuum fitting lines. Remove the line if necessary and block the port to try and isolate the offending line.
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Old 07-21-2012, 01:59 PM   #4
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Quote:
Originally Posted by focusauto01 View Post
That is strange your not getting a code from the sensor not functioning properly. My post cat sensor wasn't functioning correctly and mine threw a code. I had a rough idle. I went ahead and changed both sensors. Takes less then 30 minutes. Car runs and idles baby smooth now, and has much better throttle response.

My assumption would be that it sets a code too?? So I'm kind of confused. I would go ahead and just throw a new sensor in there considering it takes a few minutes, if your funds allow.. How many miles does the car have if you don't mind me asking?
Mileage is a little over 100K. I'm confused, too! I was considering just getting new O2 sensors, but I've played the "this has to be the fix, so I'll just buy it" so many times with this car that - wasted money aside - I'm tired of the endless cycle of anticipation and disappointment!
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Old 07-21-2012, 02:06 PM   #5
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I think you're chasing a red herring with the rear o2 sensor. It's there just to check cat efficiency; it's not part of the A/F ratio adjustment feedback loop.
In my case, an erratic idle was a vacuum leak in an odd position: Where the EVAP purge line - the large one - enters the intake manifold, there is a rubber O-ring hidden within the "twist to tighten" fitting. Wiggle the fitting and listen for a hiss or change in idle quality.
Regardless if it's that or not, re-check all your vacuum fitting lines. Remove the line if necessary and block the port to try and isolate the offending line.
That was kind of what I was thinking (the red herring part), but it was just too strange not to notice and investigate. I've posted a picture below of the live read out. If you're saying that my assumption about it being a passive sensor is correct, then why no codes.



I have tried the "spray test" with some carb cleaner to check for vacuum leaks, but there were no obvious results, and I'm a little gunshy about spraying flammable liquids all over the engine compartment! I will check into the EVAP line you mentioned.

One other clue: I had the classic collapsed PCV hose - when I replaced it, the problem actually became more pronounced...
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Old 07-21-2012, 04:35 PM   #6
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Not sure why there isn't a code either; so try to purposely invoke one. Disconnect the wiring harness connector to the rear sensor. You should get a P0420 DTC code since the PCM will no longer detect a voltage difference between the front and rear O2 sensors.
With regard to the idle problem, forgot to mention I have read of the manifold O-rings deteriorating causing vacuum leaks. You have to dig a bit to rectify those. Just tossing out some more obscure possibilities for consideration if you've tried the obvious...
And the PCV hose replacement making it worse is concerning. It's awkward to get at and position securely. Sure the repair was solid? Brake vacuum hose is nearby too.
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Old 07-22-2012, 04:09 AM   #7
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The rear O2 normally stays fairly steady, it will start to swing high and low more and more as cat dies. It CAN switch SLOWLY across the high and low volt without a code, it is only when it switches faster and faster that at some arbitrary time the PCM determines too fast and too much switching and sets DTC.
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Old 07-22-2012, 12:41 PM   #8
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The rear O2 normally stays fairly steady, it will start to swing high and low more and more as cat dies. It CAN switch SLOWLY across the high and low volt without a code, it is only when it switches faster and faster that at some arbitrary time the PCM determines too fast and too much switching and sets DTC.
Thanks, that's helpful information. Would the cat going bad effect my idle?

EDIT: Looking back at my diagnostic data, my long term fuel trim data is at 99.2%. Not sure if this is indicative of anything...

Last edited by Joboo; 07-22-2012 at 03:47 PM.
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Old 07-22-2012, 03:11 PM   #9
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With regard to the idle problem, forgot to mention I have read of the manifold O-rings deteriorating causing vacuum leaks. You have to dig a bit to rectify those. Just tossing out some more obscure possibilities for consideration if you've tried the obvious...
And the PCV hose replacement making it worse is concerning. It's awkward to get at and position securely. Sure the repair was solid? Brake vacuum hose is nearby too.
Thanks for the tips on the vac leak. Is there a catch-all test for vacuum integrity? Other than visual inspection, I'm not sure how I'm suppose to test all the various lines, or even what they are, I've had no luck finding a diagram for the Zetec. I do have a handheld vac pump, but it looks like most of the fitting are proprietary.
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Old 07-22-2012, 03:44 PM   #10
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Later style manifold is "more squarish" but fittings/functions are the same. Port marked "NA" is omitted on later style intakes.

Vacuum line manifold ports:


Vacuum line function diagram:
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