So I decided that the resistance test on the ignition wires were bad enough to justify replacing the wires. By Ohms law, low resistance will mean low voltage, which might mean weaker spark and incomplete burn, even if there is no misfire. Over a long period, that could pollute the CAT with unburned fuel. The new ignition wires showed about 10,000 ohms per foot (25K is the Honda FSM service limit). Seems to have changed functionality only modestly.
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Originally Posted by me and my metro
Can you see both O2 sensor patterns at the same time? ...
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I can now. I had to do a freeway errand tonight and forgot my scantool. On the way to, the P0420 code popped again. I set my Ultra Gauge to show both O2 sensors side-by-side before coming home. At idle the downstream mostly stayed at about 0.9v (1.25 max and rich, 0.1 min and lean). The upstream moved around pretty normally. It was on the freeway that the fun started. At steady cruise I caught the downstream sensor dip into the 0.1 voltage area once and a couple times into 0.2v. That means a lot of oxygen suddenly crossing the sensor. Clearly I have a CAT problem. But nearly all the time, the downstream stayed in the 0.8v or 0.7v range and did not seem to swing widely. So I think it is contamination and something else...
The short term fuel trim is going moderately to very negative, sometimes -25%+ but often only -7% or -9%. Longterm trim is not at -3.9%. SO I still think part of my problem might be unburned fuel getting into the CAT.
I will acid wash the CAT, but first I will test some other items looking for the cause of the rich condition.
1) Test spark plug itself.
2) Test the MAP.
3) Test the IACV.
4) Test the injectors for leaking(?)
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Originally Posted by Baltothewolf
Well do the anti fouler trick and see if it will pass smog? The cat may be on its way out, but it might be good enough to pass. You could also play with the timing a little to lower emissions to help it.
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I guess. But if I do stuff like that the underlying problems will still be there (a failing CAT and a persistent rich condition). I want to car to run well, clean, and durably. So I gotta diagnose it right, if I can.