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

I have been CEL free since the slow response time code. I have not done anything though. No changes to my setup. But then, summer heat in Arizona so intense I have not been driving it as much. When it cools down I will be driving it more ... so we’ll see how it goes.

Steve

 

Aug 25 and still going good. Seems like the P0421 and P0431 codes are a thing of the past. Hope that continues. Have not had a recurrence of the “slow response time “ code. We’ll see how that goes as the weather cools down here and I drive it more. Lastly, had the emissions testing yesterday and passed. I did not expect otherwise, but was concerned none the less.

 

Driving home last night and got the P0139 / P0159 codes again. Damn! I thought I had this resolved.

So this is my set up ...

Fabspeed headers (no pre cats)
Stock Cats

Initially, was getting P0421 / P0431 codes ... catalytic converter efficiency. So installed extenders and mini-cats. Resolved those codes then started getting P0139/P0159 codes ... Downstream O2 sensor slow response time.

Tried various things included 24 hour reset of ECU by turning power off. I replaced the downstream O2 sensors. I had both extenders and mini-cats installed so removed the extender and went with mini-cats only. The P0139 / P0159 codes remained off for longer time now ... like 6 months or so and I had driven quite a few miles.

So last night, got the codes back. Slow response time on O2 sensor. Going to try to remove the mini-cats and put in extenders only as the mini-cats are more restrictive than an extender only. I do not have high expectations that I am going to resolve this.

Steve

 

Hi 360trev,

When you apply the fix using your Swiss Army Knife, will this cause a "not ready" condition when taking the car for emissions testing? Some ECU tunes that ignore certain codes will then cause that condition to be flagged when going for emissions.

Steve

 

I have been chasing this gremlin for some time now and I believe I may have it resolved. First a bit of history ...

I installed Fabspeed headers which eliminated the "pre-cats" in the stock headers. Soon after, I received Check Engine Light (CEL) code number … P0421/P0431 … which, I believe, are interpreted as “Warm Up Catalyst Efficiency Below Threshold”. More than likely caused by less catalyst material between the upstream O2 sensor and the downstream O2 sensor.

I purchased and installed two Big Daddy mini-cats and installed them in the bung located on the catalytic converter.

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This resolved the P0421/P0431 codes for a while, but then I received another CEL, this time with codes P0139 / P0159 … “O2 Sensor Circuit Slow Response”. What this is saying is that the O2 sensor(s) are not responding as fast as the ECU would like it to.

My understanding is that the front O2 sensor will always be fluctuating its voltage output and the rear sensor is expected to have a lower output. The ECU is expecting that the voltage between the two to be greater than some threshold. If it is not, then it assumes that the catalytic converter is not operating efficiently. The ECU is also looking that the rear O2 sensor responds to fluctuations in voltage in synchronization with the front ECU. If it measures too long a lag time, it assumes the rear O2 sensor is bad. Or maybe an exhaust leak, wiring problems or other issues.

So, a common resolution to bring the O2 sensors in line with each other is to:

1. Install a min-cat extender and install the rear O2 sensor to reduce the output voltage of that O2 sensor .... or ....
2. Install a simple extender for the rear O2 sensor to pull it out of the exhaust stream to accomplish the same function … i.e., reduce is output voltage so the ECU thinks the catalytic converter is working OK.

As mentioned above, I tried the mini-cat approach and that resolved the “catalytic converter” not working indication, but introduced the secondary problem of slow O2 response time.

I then decided to go the route of simple extenders. I got a couple of these …
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But, I still got the P0139/P0159 codes. Slow response times. So decided to get shorter extenders. I got a couple of these which are called "spark plug anti-foulers"
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Discovered the throat depth too short to insert the O2 sensor. The O2 sensor needs more throat depth to allow it to be threaded into the extender. So enlarged the inlet hole like this …

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That resolved that “slow response time” code, but re-introduced the “inefficient catalytic converter code” The shorter extender put the O2 sensor too far back into the exhaust stream.

So, I went back the longer O2 sensor. This time enlarging the hole in the extenders. I still got the “slow response time” codes. Removed the extenders and made the inlet a bit larger. Now this time I only received the code on one bank. The other bank was OK.

Remove the extenders again and now enlarged the holes another 1.5 mm (1/16”). I think I now have found the sweet spot. Went for 200 mile drive yesterday and all seems well.

The end result is that I now have two O2 extenders with the inlet holes enlarged to 10.5 mm (13/32”). If need be, I could open up the inlet a bit more.

Now, let me tell you that it is a royal pita incrementally enlarging the extenders. I started with this …
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And kept incrementally enlarged the hole until I did not get the “slow response time” codes. I hope I am done. But, now I have found this extender on Amazon.

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This gives you one extender with a 12.6 mm orifice and three inserts that have 8.6 mm, 6.2 mm and 3.6 mm orifice. The C-ring secures the insert in place in the end of the extender. I wish I had this at the start.

To summarize … if you are changing out your stock headers on your 360 and you end up getting “catalytic converter efficiency codes”, I would recommend moseying over to Amazon get two of these extenders …

https://www.amazon.com/gp/product/B07F7QFBGQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1


Starting with the smallest orifice, install the O2 sensor and see if that resolves your CEL codes. With the smallest orifice, you should not be getting the “inefficient catalytic converter code”. But, if you get the “slow response time code”, then try the next larger orifice. Keep enlarging the orifice until the codes are eliminated. My process of enlarging the orifice on the drill press works too, but much more of a pain.

Hope this write-up helps others.

Steve

 

Update on chasing the P0421 and P0431 codes ...

I am pleased to say that two months have passed since I installed the two O2 extenders with enlarged holes for exhaust gas flow ... no issues, no check engine lights. I have put on about 400 miles since May and no problems at all.

I have been chasing this problem since installing the Fabspeed headers, which eliminated the pre-cats that were in the stock headers. Go back and read through this thread on my trials and tribulations. The solution seems to be the installation of two O2 extenders with the exhaust gas holes enlarged to about 13/32" ... The size of the hole is not an exact science. Achieved by trial and error. My guess is the hole could be 7/16" to 1/2" and achieve same result (approx 11mm to 12.5mm diameter). This reduces the restriction in gas flow so you avoid the P0139 / P0159 -- "O2 sensor slow response time" error, yet pulls the O2 sensor far enough out of the gas flow to avoid the P0421 / P0431 -- "Warm Up Catalyst Efficiency Below Threshold" code.

The easiest way to achieve the optimum size hole in the O2 extenders is to purchase this item on Amazon ... https://www.amazon.com/dp/B07F7QFBGQ?tag=vglnk-c31-20

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... which has three inserts that will provide quick and easy adjustment of the gas flow opening. Changing inserts is quick and easy versus drilling out the extender itself. With the three inserts you have four different size openings that you can configure. I really like the ability to change the orifice size as the optimum size may not be the same for every car ... in fact, I would expect that the optimum size is different for each car due to slight differences in catalytic converter efficiencies, fuel trim and many other factors that can affect the operation of the O2 sensors.

If you are installing new headers or going to do a cat-delete, I would definitely try this route before spending big bucks on mini-cats or an ECU tune. I went the mini-cat approach at first and it was throwing money down the drain ... for me. Some people have been successful with the mini-cat approach, but I was not. This way has given me positive results and had I done this at first, I would have saved a couple hundred $$ and many hours of time.

I am hoping that these gremlins do not come back. So far ... so good.

Steve