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Discussion Starter #1
So, after 331K, the post-cat O2 sensors are finally throwing faults (surprisingly, in unison) in my 325Ci.

Firstly, can I safely assume that the post-cat sensors on an E46 have no effect on performance, and are there solely for emission control?

If so, assuming the heater circuits are still working but the sensor portions aren't, is it easy or hard to trick the ECU with a few simple passive electronic components, like resistors and diodes, to get the ECU to think the sensor is there and working correctly when it's actually failed?

As I understand, and I'm not quite 100% clear as to what's happening here, the ECU is sending a fixed 0.45 volt signal to the sensors, and either they or the ECU is fluctuating the same signal line (when working correctly) from about 0.2 to 0.8 volts about once per second.

This must mean either A: the ECU is sending a "weak" current, which the sensors are then able to alter based on sinking their own amounts of current, or B: the ECU itself is cycling the voltage from low to high over the entire range.

Am I also correct that the ECU is monitoring not only how many times they cycle per second, but how high and low they are able to change the signal voltage?

If this is all correct, just intuitively it would seem fairly complex to go about simply tricking the ECU with a few passive components. Or is it?


Thanks for any clarification.
-Dewey
 

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The post-cat sensors monitor the performance of the cats. If they are throwing codes your cats may have chased one too many mice. In English: your cats may be bad.

I assume WA requires bi-annual emissions inspections, so you can either replace the cat$$$ or fool the ecu.
 

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So, after 331K, the post-cat O2 sensors are finally throwing faults (surprisingly, in unison) in my 325Ci.
Well this is ultra vague!

So are the code(s) related to the O2 sensors or are the code(s) related to something else? We kind of need the actual Pxxxx codes along with Freeze Frame data to get a better idea about what is going on.

I RARELY ever see O2 sensor codes for anything other than failed heater circuits. I condemn and replace O2 sensors all the time that never trigger a code but are lazy and do not perform well.

Also in case you were unaware, O2 sensors are "consumable" so they do not last forever. It is recommended they be replaced ever 100k miles or earlier if there is a problem/fault.

If we are dealing with P0420/P0430 codes, these are NOT typically O2 sensor errors.

Once we can determine what the problem is, if the O2 sensor(s) need to be replaced, why not just replace them if necessary rather than trying to spoof or hack something?

You should be able to get Bosch direct fit O2 sensors for around $50 each if you shop them and find some coupons.

But like anything, lets start with some facts and data before jumping in head first.
 

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I second the replacement of the sensors if needed instead of trying to find a work around.
 

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Plenty of workarounds including EU2 DME flash, O2 Emulation, etc. They are only worth it if you MUST remove cats due to having some 18 PSI turbo, supercharger or other really expensive real performance mods done, otherwise just replace the sensors and drive like a normal person.
 

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You can buy simulators to fool the ECU on cat performance but the ones I've seen require the O2s left in place and have functional heaters. But it's probably easier to just replace them if the cats are good.
 

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Discussion Starter #7
Yeah, I thought it would probably be easier to just replace them. I just figured if it would be as simple as soldering a resistor inline with a wire, and it wouldn't hurt the engine, hey, I would do it to save $100.

Well, I can't tell you guys what the codes were just yet, because I thought I'd be clever and do something that worked once before...I used a code scanner to clear the codes, went to emissions (yep, I'm in WA, where it's every 2 years), and failed due to numerous systems reporting "not ready". When I did it before, it passed, so it must have been an intermittent error that I got lucky on when it didn't come back once the systems were ready last time. This time, as soon as some of them reported ready, they failed (an unrelated error to the O2 sensors - secondary air system codes). After almost 2 days of driving, the system relating to the oxygen sensors still isn't ready, so I don't have the codes yet. I imagine as soon as it becomes ready, it will report what it has before. I don't recall the exact numbers, but it was the same as when the pre-cat sensors went - something similar to "O2 Sensor, Bank 2, Sensor 1".

I'll report back as soon as they become ready and I can confirm the codes.

Thanks for the feedback so far.

-Dewey
 

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A "good" car will clear the Readiness Monitors in at least 2 run cycles and 15-25 miles of driving as long as about 10 miles of the driving is at 45-55 highway cruising.

If you get a smart phone/tablet OBDII App and interface, you can determine what exactly the O2 sensor health is.

See these links -

http://forum.e46fanatics.com/showthread.php?p=16203126

http://e46fanatics.com/forum/showpost.php?p=16237779&postcount=34

http://e46fanatics.com/forum/showthread.php?p=16332904

http://e46fanatics.com/forum/showthread.php?t=1051941

http://e46fanatics.com/forum/showthread.php?t=1050205
 
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