P0421 / P0431 Trouble Codes (Catalytic Converter) ... anyone have good fix?

They install in rear

 

I believe you answered your question. The front O2 sensors are in the exhaust stream before any cats. They sample the exhaust gases immediately after engine manifold and are used by the ECU for stoichiometric analysis so that the ECU can dynamically make adjustments to fuel trim, etc.

The rear sensors are only used to determine if the cats are working properly. When you removed the stock headers with precats, the rear O2 sensors are now thinking the cats are not up to snuff. By inserting the mini cats your goal is to restore enough catalytic material in front of the rear O2 sensors to make those bad boys behave themselves.

 

When you remove the stock headers from the 360, there is less catalytic material between the front and rear O2 sensors. This is root cause of the CEL codes being generated. The rear sensor output voltage is such that the ECU is sensing that the catalytic converter is not up to snuff.

I invite you to look at the comparison between the 360 and F430 headers and catalytic converters ...
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You can see that between the front and rear sensor there is an amount of catalytic material in both the headers and front portion of the 360's catalytic converters. When you replace the 360 headers, you lose some of that catalytic material. The F430 headers do not have any catalytic material. Therefore replacing the F430 headers with aftermarket headers will not affect the amount of catalytic material between the two sensors.

What those of us with the 360 have tried doing is to add mini cats at the rear O2 sensor position and then install the rear sensor on the end of the mini-cats. This is trying to restore the amount of catalytic material the exhaust gas passes through before the O2 sensor ... hopefully to get the O2 sensor's reading back where it was when the stock headers were installed. This generally works as one would expect. However, there is a secondary problem that is generated that deals with the response time.

On the 360, the ECU not only looks at the voltage output of the rear sensor, but it also checks the timing of voltage spikes that occur with both front and rear sensors. The ECU threshold wants see that when the voltage spikes on the front that there is a corresponding spike at the rear sensor. If the timing between these spikes is too long, then this will trigger a "slow response" time CEL. When you install the mini-cats, this is a common problem.

So, how to resolve this issue?

I have found one solution that seems to be holding up now for several months. I removed the mini-cats and installed a pair of O2 extenders. It is worth noting that when you purchase O2 extenders, they typically have fairly small orifice for passing the exhaust gas. This orifice is too small and you will still get the CEL for slow response time. You need to open up that orifice by drilling out the body of the extender. But, an easier solution is purchase O2 extenders like this ... available on Amazon.

https://www.amazon.com/dp/B07F7QFBGQ?tag=vglnk-c31-20
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In my case, I am using the largest orifice available with the above extenders, but the nice thing is you can easily increment and select four different size orifices using the three inserts. In my case, I am not using an insert.

In my opinion, O2 extenders are not optimal, but so far they are working for me. If I do end up getting a slow response time code, it is an easy matter to read the code and clear it.

A more optimal solution is to restore the amount of catalytic material between the front and rear sensors. Taking a cue from the F430 system, if one were to move the rear sensor to the downstream end of the catalytic converter, this would then restore the amount of catalytic material between the sensors. It would also dispense with the need for any mini-cats or O2 extenders. This can be accomplished by plugging the existing bung on the catalytic converter and installing a new bung as shown here ...
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I have not tried this, but I have examined the cats on my 360 and this looks perfectly doable. The rear O2 sensor have enough length of wire to accommodate re-positioning them as shown above.

If I had known this at the start of my odyssey, I would have done this first and would not be messing around with mini-cats and O2 extenders. But, I am the point now with the larger orifice extenders which are yielding good results for me.

Another solution is to reprogram the ECU to ignore CEL codes. I expect this solution will also provide very satisfactory results but I expect the cost is likely higher than moving the rear O2 sensor. It certainly is more costly than the O2 extenders I explained above.

Steve

 

I still have the stock catalytic converters and the Fabspeed headers (without the pre-cats) and using the O2 spacers I mentioned in my earlier post ... the CEL I was getting has gone away. Been about several months and no issues. That is not to say that the CEL will not come back. But, if it does, I will move the rear O2 sensor further back in the exhaust flow.

I have heard of people wiring in a small resistor into the O2 sensor lead. If memory serves me, it was a 51 ohm resistor wiring into the black lead on the O2 sensor. But, I would not be surprised to learn that this method might clear the CEL about low efficiency, but might not clear the CEL about slow response time.

I would try getting these adjustable gas flow O2 sensor spacers ...
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You can purchase these on Amazon. Here is the link:
https://www.amazon.com/EVIL-ENERGY-Restrictor-Fitting-Adjustable/dp/B07F7QFBGQ/ref=sr_1_5?crid=1MA1SYM36M10G&dchild=1&keywords=o2+sensor+spacer&qid=1589293498&sprefix=o2+,aps,190&sr=8-5


They are inexpensive and I found that using two of these without the inserts yields the largest size orifice for maximum gas flow and my CELs are gone. Cannot definitely say they will fix your situation as your mileage may vary. But, worth a try for less than $25 for two of them.

Steve

 

Also on Amazon, is this adjustable flow O2 sensor spacer that is curved and looks like it results in a longer flow for the exhaust gas. There are two types of CEL codes that one is likely to get coming from the downstream O2 sensor:

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1. O2 sensor efficiency code. Caused by a missing O2 sensor or having the O2 sensor too far out of the exhaust stream. Interpretation is that the sensor efficiency is bad and would normally lead one to replace the sensor.
2. O2 sensor slow response time. The normal interpretation is that the sensor is bad. This is caused by too little exhaust gas reaching the sensor and the car's ECU is looking for spikes in voltage at the rear sensor to be in sync with spikes on the front sensor.

The problem is trying to balance the two. In this balancing act, I found the adjustable gas flow sensor spacers to work for me.

Here is the link for these curved spacers ...

https://www.amazon.com/Vibrant-11619-J-Style-Restrictor-Adjustable/dp/B00AQ7972O/ref=psdc_15730891_t5_B08562QDW2

Steve