Diagnosing a P0420 code

[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.

[California98Civic]

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.

Originally Posted by me and my metro

Can you see both O2 sensor patterns at the same time? ...

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(?)

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.

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.

[California98Civic]

Lunch break testing of the spark plugs.

The plugs in the car are shot. I did not realize how badly the contamination they show might errode function. With great difficulty getting any resistance readings, I eventualy managed to get these: at the 20k setting on my meter I got #1 @ 8.60, #2 8.65, #3 8.75, #4 9.17... but they all failed the ground test and the continuity test because of corrosion. They were always a variant of the stock plug anyway.

I have a lightly used set of the stock plugs on hand for smog testing. They passed continuity and ground tests and I got these resistance readings with relative ease when they were still a little dirty:
#1 8.75, #2 9.59, #3 9.80, #4 9.66

So, I cleaned them to shine and bare metal and gapped them to 1.0mm to 1.1mm.

Will see how they do on my errand with my kid tonight. I fully expect to get the P0420 again. But solving all this in the fuel/air systems seems important before I clean the CAT and see if I can regain acceptable emissions efficiencies.

[California98Civic]

Thanks for all the input guys. The freeway errand tonight lit the P0420 again on the way out. This time I was watching the upstream and downstream 02 data on my UltraGauge and when the light went off I was actively expecting it because the two readings began to move almost in unison for a few seconds. On the way home on the freeway, there was a prolonged period of 5 seconds, 10 seconds, or more when I could see the two readings moving almost in unison. I got lean readings from the downstream sensor as lean as 0.15 volts repeatedly.

I am gonna start a new thread on the persistent rich condition. Once I get a better understanding of the rich condition, I will clean the CAT with an acid wash.

Thx!

[California98Civic]

Got a nice surprise.

I don't know if it will last, but suddenly the rich condition was gone on my return freeway run last night.

To recap, I tested and replaced failing ignition wires and spark plugs but the very high STFT readings (as high as -29) continued at idle freeway steady cruise across two drive cycles. I did my freeway cruising using P&G last night (third drive cycle) on the way out to appointment and the LTFT, as expected, improved from -9.4 to -5.4 as I expected. That's normal LTFT behavior for P&G. On the way home I wanted to see the negative fuel trims again and observe, so I steady cruised it, but all STFT readings were normal.

It seems possible that the ECU has "learned" it faces normal conditions now. But I am gonna wait a few more drive cycles and use the sensors for diagnostics. Maybe an intermittent failure has gone away temporarily.

The downstream O2 sensor reading is still fluctuating a lot, though, so I still think I need to clean or replace the CAT.

[California98Civic]

Tonight's two drive cycles at steady cruise freeway speeds makes three in a row without a hint of the rich condition, super lean LTFT. In fact LTFT is at 0.00 exactly now. Better yet, I have not seen the P0420 return, either. This exact drive has set that code off at least 6 or 8 times this month. I wonder if that will continue.

[California98Civic]

On fifth drive cycle since fuel trim fix, the P0420 came back today. Diagnosis testing done. I have an inefficient CAT. I will clean it to see if that clears the code. I'll post those results in my CAT restoration thread.

At least this process taught me new testing techniques and revealed a simple cause for the bad fuel trims.