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Discussion Starter #1
I'm trying to smoke out a pesky P0420 code. I can clear it but it will come back like clockwork an about 110 mi.

If I run the engine at a steady 2300 RPM, the voltage output of the upstream o2 sensor pulses nicely between .1 and .9v like it should, and the downstream sensor is also pulsing a little but at a much lower voltage, .01-.02v. So far, so good. However, every 5 seconds or so, the the downstream sensor suddenly goes nuts and pulses a few times with a much higher voltage very much like the upstream one does, then suddenly behaves nicely again. And every 15 seconds or so it goes nuts again, but keeps doing this much longer (say,5 seconds or so) before settling down again.

Two questions:
1. Is it possible for the converter to fail in such an intermittent way that a good o2 sensor would respond that way?

2. Is it possible for an o2 sensor to fail by producing an exaggerated voltage in the intermittent way that I'm seeing.

Thanks for helping!
 

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...the upstream o2 sensor pulses nicely between .1 and .9v like it should, and the downstream sensor is also pulsing a little but at a much lower voltage, .01-.02v. So far, so good.
No, not good, so far. Upstream behavior sounds ok, but the down stream O2 sensor should not behave like that; it should stay in the .5Vdc range with slow hunting, slow peaks, slow valleys, slow as compared to the upstream sensor. Running off of my memory atm, which is dangerous. Need someone else to confirm, gotta go.

If me, I would replace the down stream sensor.


Two questions:
1. Is it possible for the converter to fail in such an intermittent way that a good o2 sensor would respond that way?

2. Is it possible for an o2 sensor to fail by producing an exaggerated voltage in the intermittent way that I'm seeing.

Thanks for helping!
1. dunno. guessing no.
2. dunno. guessing the low voltage periods are the failure, are the problem. The higher "going nuts" readings are the desired ones.
 

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Grey Friar
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Since it looks like you autoX, how about just get a Xcal tune and ignore the rear O2 signal... a touch more performance that way.
 

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Discussion Starter #5
Since it looks like you autoX, how about just get a Xcal tune and ignore the rear O2 signal... a touch more performance that way.
Yes, I thought about doing something like that even before this started happening, but I never acted on it. It just seems that when you start down that path there's always side effects, like pesky trouble codes!

The car has 120k on the clock and both the o2 sensors are original, so I think I'll try replacing that rear one. If I'm wrong, well, it was probably almost dead anyway...

BTW- thanks to the three of you for your responses.I'll let you know how this works out.
 

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Discussion Starter #6
guessing the low voltage periods are the failure, are the problem. The higher "going nuts" readings are the desired ones.
Turns out this is correct. I changed the downstream sensor today and now it's output isn't much different than that of the upstream one. New converter on the way.
 
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