I'm Back! (P0421 CEL Code)

[Navysteveo87]

So I'm back and the problem I'm now having is CEL code P0421. (Paragraph #5 of my original post below is what I think is causing the issue)

https://mirageforum.com/forum/showth...ctrical-Issues

I'm having the same exact issue as littlemoule had to a T. (Cylinder 1 was misfiring (for 2 weeks) and dumping fuel in the cylinder that wasn't getting consumed, it's possible the raw fuel being pushed into the catalyst could have degraded the efficiency/lifespan of the catalyst)

https://mirageforum.com/forum/showth...705-P0421-code

After Johnny Mullet fixed my corroded wiring harness back in February the car has been running great but the mpg's are now suffering (was getting 37-40 mpg highway prior...currently maybe 30-33 mpg?) and now the CEL (P0421) has been on since

I've tried the old Scotty Kilmer lacquer thinner in the gas tank trick to clean the cat but that did nothing.

https://www.youtube.com/watch?v=5icT...eSBraWxtZXI%3D

I just purchased two O2 sensor spacer/defoulers to try and trick the computer into turning off the CEL. Before I try it has that worked for anyone else before or should I just remove the front cat and clean it out real good with soap/water?

https://www.walmart.com/ip/2Pcs-O2-S...ole/1093527612

Also, How many O2 sensor's does the 2015 US Mirage have, two or three? Thanks

https://www.youtube.com/watch?v=0NM55ar7GSE

[Navysteveo87]

Got it. I'm not sure why there is a second catalytic converter on the car if there is no O2 sensor after it. That's what was confusing me. (I thought a third O2 sensor would go after the second cat but that's not the case)

So in theory I could remove the second cat (and run a straight pipe in its place) and not throw a CEL and pass emission inspection (e-check here in Ohio) because I already have a cat off the exhaust manifold when they do their visual check.

Ok so back to my question. Can I remove the first catalytic converter (from the manifold) and clean it out by soaking it in hot soapy water? Or am I SOL and just have to buy a new $300 cat off rockauto? If I don't want to do either (and just want the CEL to go away) can I install the O2 sensor spacer/defouler on the second (downstream) O2 sensor?

[Eggman]

1. The first catalytic converter (often called a pre-cat) is a smaller converter located close to the exhaust manifold in order to heat up faster. Once this pre-cat heats up, it in turn helps to heat up the main catalytic converter.

More details can be found here: Catalytic converter. Refer to the section titled "Placement of catalytic converters"

2. Would you please further explain using a soak in 'hot soapy water' to clean catalytic converters? Sounds interesting.

[Eggman]

The front cat is the primary not the smaller rear cat. I can see how this can be confusing.

You're the expert!

[Navysteveo87]

1:50 mark

https://www.youtube.com/watch?v=5icT...eSBraWxtZXI%3D

1. The first catalytic converter (often called a pre-cat) is a smaller converter located close to the exhaust manifold in order to heat up faster. Once this pre-cat heats up, it in turn helps to heat up the main catalytic converter.

More details can be found here: Catalytic converter. Refer to the section titled "Placement of catalytic converters"

2. Would you please further explain using a soak in 'hot soapy water' to clean catalytic converters? Sounds interesting.

[Eggman]

1:50 mark

https://www.youtube.com/watch?v=5icT...eSBraWxtZXI%3D

Wow, that Scotty Kilmer sure knows his stuff. I would never think of trying soapy water to clean out a catalytic converter. But he's the expert!

[Johnny Mullet]

You can try cleaning it, but I really never heard of this being done. Remember, you been running on a missfire for many, many miles and I would assume the cat is done for. You can try removing and then gutting the cat or both, but you will get a CEL, but it might not affect performance.

[Navysteveo87]

Clean it like this?

https://www.youtube.com/watch?v=3DsUWWs2l6k

On another note the car has been running really good. I couldnt tell that the cat is clogged other than getting poor gas mileage

Also, If I install that O2 spacer will that clear my CEL?

[Johnny Mullet]

The spacer may or may not clear the code. A blocked cat is your main issue.

[7milesout]

Steve - Lots of good info, and a good thread. It's also interesting because this is not as simple as it seems. Let me preface my comments by saying that I am not the expert here. I can offer some rational maybe, but might be about the limit.

1. I don't know this, but it would seem that whichever cat has a sensor before AND after it, would be the main cat. Which makes me wonder why the 2nd cat needs to be there. Which also makes me wonder, what my car would sound like without the 2nd cat. I want it just a touch louder, but not loud. It's possible I could yank off the 2nd cat and not get a check engine light, and sound like an 8V72? I'll have to think more about this later.

2. O2 spacer. I've had some experience with this. I bought a $50 "minicat" from something like Big Daddies (seems like). And it did nothing. I still got the check engine light. This was on my son's RX8 with a failed cat (that was removed and replaced with a double resonator straight pipe). The RX8s tend to go through cats, and the added heat on the wankel due to a cat being there (especially one that is bad) causes Wankels to go bad too. So we decided to try to just delete the cat. Anywho, since the first crap mini-cat wasn't working, I found a <$10 one on Walmart and decided to give it a try because it was so cheap. The $10 worked like a friggin charm. He's put thousands of miles on the car with that cheap mini-cat on it and the CEL & code has never returned.

If my car were having the problem you are having with your car, I'd be very tempted to try to run the car catless (if the cat cleaning didn't work), but with the mini-cat. I would have warranty issues. However, the way the Magnusson-Moss Act reads is, I would be losing warranty only on the exhaust portion of my drivetrain warranty. I could care less about the cat, but I would rather retain an unquestionable warranty for the duration. Of course now that I think about it, I doubt the exhaust is covered under the drivetrain warranty anyway.