BHR midpipe and hjs cat calls up p0428 CEL
 

P0420, not 428 the title was a typo

Was going through the issue in this thread

https://www.rx8club.com/series-ii-te...s-tool-267088/

Decided this new title would catch the attention of people with expertise on this :


I spent some time reading about this before going for that combination, and my initial concern even before I bought it was the CEL going off. I put it on the car, drives beautifully with a lot more torque. But one week after getting it in the CEL comes on, I think no one is surprised. I get the CEL reset and it returns 2 days later.

I payed about 600 for it lightly used from this forum, the cat and pipe welded, so I'm sure the cat is fine. Am I stuck just downloading the torque app and resetting the CEL once in a while myself since there's probably nothing going on at all. For example before emissions just resetting it? Obviously I'd have to check once in a while to see if the CEL is pulling legit codes.

Thanks

That may work if the emissions test is limited to looking at the dashboard to see if there's a CEL.

If your emissions test is based on OBD-II plug-in then resetting the p-code may not help you pass. The PCM records this and the emissions readiness monitors will report that to the emissions tester if you recently reset it.

Either your cat or O2 sensor are probably bad.

You can use something like mazdaEdit or Versatuner to mask the code. People have reported that their cars pass plug-in emissions testing this way.

Step 1 is to know what the code is. It may have nothing to do with the midpipe. Considering it took a week to show up AND you had a pre-existing CEL. What problem was the HJS cat meant to solve?

Resetting it via the app is not a solution, the emissions people will know you reset it if they access the engine computer. Mask it as mentioned above or figure out what the actual problem is and address it.

My original oem cat died, was starting to kill the engine so I needed to replace it, it's just coincidence that some one was selling an affordable cat which was attached to a midpipe.

How much in terms of labor would normally be required to figure out the actual cause? After all the expenses recently I'm not crazy about getting more.

Reading the code is free at most places. Once you know what actual code it's throwing, we can talk about solutions. That cat didn't die by itself, so is it possible your ignition coils are old?

Oh I had the code read at the Mazda dealership on Monday, I know it's the p0420 for sure. I replaced coils, wires and spark plugs when I did the cat too. And yes the old ignition parts contributed to the cat loss but we're talking 160k, so I get the oem cat is very well put together, but I'd be surprised if it lasted longer.

You bought a used cat, it's possible it's clogged enough to throw a code.
They can get damaged by misfires pretty quick.
Do you know why the owner sold it?

Well the vendor made this thread a little while back indicating why, there's pics of the pipe there

https://www.rx8club.com/rx-8-parts-s...s-sale-266150/

And this is the thread I got the sale off, if it matters. He edited the thread to remove the images and obo


https://www.rx8club.com/rx-8-parts-s...js-cat-266666/


Or should I ask him?

Guessing doesn't get you far. Actually get the code read.

If your previous cat failed due to failing ignition components, there is a chance that the rear O2 is damaged.

Do you know the HJS part number on your cat?

Do you live in Ontario? I recall that's the only province in Canada that still has emission tests, and according to someone who lives in Ontario on the Accord forum I go to, it's "just a farce", so I am not sure how strict they are.

So there's 3 next steps I see. See if the previous owner also had CEL with that cat. If yes then look into masking it with MazdaEdit, or another cat.

If not then either replace the O2 sensor outright, or bring it to someone to look at the actual voltage signal from the sensor, and use that to determine if the sensor is sane. It might be cheaper to replace it than diagnose and then replace, but the flipside is that the sensor may not be the problem. The diagnosis should also give you cat temperature which will tell you how healthy it is. 900C is normal. 1000+ is too much.

Make sense?

there's a reason the service manual gives you a list of procedures to follow for troubleshooting a code

Thank you for the insight and advice.

So there is more information than just p0420 that the scan reads ?

I already talked to the previous owner, he said he had no CEL.

Which it did in part or mostly. So one of the O2 sensors is one that I had from before and still there? There is 2 of them then, the front one is attached to the new cat, so that my understanding is clear.

If that is the case probably replacing the sensor appears correct. I see them online for less than a 100, simple install in guessing.

Is it fair to think checking the temp on the cat is not a big project?

I can check to see what the past number is, if he still has it. Probably does.


Yeah I did emissions 2 years ago, the guys doing it didn't look particularly serious about it, but that's making an assumption.

Wait. You say here the code is p0420, but in the title it's p0428. Those are different, 428 clearly points to a sensor failure. So which is it?

The 0428 is a very stupid typo, I should apologize for that. It's actually p0420.

Assuming the OP doesn't have the manual, do you have a link for it?

He doesn’t have even the basic tools so I certainly don’t expect him to have a service manual. Like I said in the other thread, you need someone with the tools and knowledge to solve this for you because you don’t have these things for yourself. Not trying to be condescending etc. It’s just the honest truth and, other than just guessing and replacing everything, that’s what it will take to logically work through this and effectively resolve it. It’s basic problem solving 101 and Mazda lays out the procedure to follow and work through it.

If he's motivated and willing to learn, and has a place to work on it, then he could download the manual and buy a few tools needed to fix it.
He can probably buy everything needed including a jack and stands for what a dealer or shop would charge him to fix it.
This is a common problem, it shouldn't be a major ordeal.
We all started somewhere.

I'm not taking anything as condescending dw, I know my place and my learning could get annoying for people, I get it . People on here know a hell of a lot, I'm still trying to figure out how many pistons my 8 has lol

Yes I saw the manuals, it's good to have. For the time being I brought it to the dealer to quickly look, he tested the cat and said it's just not doing exactly as it should, but it's clearly fine. Those were his words, as I said the car drives really great.

He also said if I want right before emissions he can clear whatever traces of the CEL. He was extremely specific with how he'd do, said he does it for people all the time.

Yea if like to get to the bottom of the issue starting with the exact part number for the cat.

I do work 7 days a week so learning to do that stuff and buying tools would be good but I'm not sure about right yet. Seems like an interesting thing to do however at some point.