It’s now throwing a P0420 & it comes back soon after clearing it. Do y’all think I’m looking at a new cat? Stinks.

 

P0420 and P0430 (same code, other bank) are common on the 2000-'02 LS1 F-bodies. GM actually issued a TSB and warranty extension for this, but that extension has long since expired. The issue is that the coatings on the substrate of the catalyst wear off, thus reducing the cat's ability to "scrub" the exhaust. The end result is increased tail pipe emissions, but that's really all. My '02 Z28 started doing this right after the warranty extension expired so I just disabled the code and went on with life, and no negative operational issues ever arose; that's the simplest fix if you don't have an actual sniffer test (where they still put a wand in the tail pipe) to pass. You can easily pass OBD scan-only emissions tests with O2 sims or proper custom tuning (a cheap, basic tune can take care of this).

 

It’s not running or idling like it has a vac leak but could a vac leak cause that code? The last thing that I did was clean the TB & MAF. Maybe one of the couplers isn’t on very well, but it’s not stumbling or anything. Do my fuel trims look normal? If it turns out to be a cat a tune & a new cat would prob cost about the same. If it indeed is the cat, you don’t think it’ll break down and clog?

 

I don't see anything in your displayed data that would indicate a vacuum leak; fuel trims look great actually (all very close and slightly negative - can't really ask for better than that).

I suppose a rear O2 sensor that is failed or fouled in a certain way might trip this as well, but the substrate coating (failing) issue is known and documented by GM so that's the most likely cause IMO - but I'm sure some folks have found a bad O2 sensor at the root of this on at least some occasions (less likely without any other codes present, though I suppose you could swap rear O2 sensors and see if the code stays the same or becomes a P0430).

Thing is, both cats are subject to this same type of failure on the 2000-'02 cars, so if you go the route of replacing the cat then you're committed to possibly having to replace the other one if/when you start getting a P0430 (same as P0420 but for the other bank) - at that point, it would have been cheaper and easier to just use O2 sims or get a basic tune and delete both rear O2s.

The issue doesn't usually involve the substrate breaking apart, though I suppose that's probably happened to someone at some point (power reduction and multiple drivability issues will be present and noticeable if there is a blockage). For the most part, it's just the coatings that wear off and cause an increase in tail pipe emissions. I left mine alone for many years and many thousands of miles, no problems at all with the cat ever breaking up or clogging.

 

One more quick question. After I cleared the code it came back on within 10 miles or so. Back in the day my wife’s Ford Expedition would get a cat code but after clearing it would stay gone for a little while. I kinda read the same about the LS1’s where people would clear it and it would stay gone for a say a month. When your cat went bad would the light come on right away after clearing it?

 

Has anyone successfully used “high flow” cats without getting the P0420/430? I have a set of these generic universal high flows (short cats is what they should be called) on my Mustang and it passed the sniffer with them, but it’s OBDI so I’m not sure if they’d be sufficient enough to not trigger a CEL. If I knew they’d work, meaning no engine light, I’d have those bad-boys welded in along with a new muffler and I’d call it a day. Also, I cleared the light and then drove it about ten miles and it didn’t come on. A few hours later I took it for a quick spin and it didn’t come on that time either. Maybe it didn’t run long enough either time. We shall see.

 

Honestly, this happened so many years ago that I don't remember how quickly the code returned. But, I wasn't particularly concerned if it was a bad cat or a bad O2 sensor, either way I was just going to disable the code. So I plugged in a pair of old O2 simulators that I had (from the days before custom tuning was so readily available, though I think O2 sims can still be purchased) and never gave it another thought. That was back in the early 2010s; I kept that car for several more years and 5-digits worth of mileage and never had any other problems from this. The only purpose for the rear O2s is monitoring the cats, so removing/disabling/ignoring them won't cause any other issues. I just didn't want to have a permanent SES light due to emissions tests (plus if the light is always on, then you don't know if some other code ever pops up unless you're regularly checking). Nothing ever changed with MPG, power, or anything else relating to driveability.

 

I know the rear O2’s are just monitors. When I was a kid I gutted a cat and lived with a CEL. My only concern is if the cat is indeed bad it could break apart and clog. Not long ago I had to replace the cats on my mustang because it was running rich for sometime and the fuel destroyed the cat and caused it to clog. It wouldn’t idle once at operating temp, but it’d run on down the road. I didn’t know the cat was causing the problem until I saw it smoking one day. OBDI, no monitors. So I replaced them with those generic high flows I linked previous. It’d be the pits to try those high-flows only to still have a cel. It’d prob sound a tad louder though.
*Edit*
I just looked and a Frost mail tune would is $175 so that’d be the cheapest route to conquer the light. Which would be a win as long as the cat doesn’t crumble & clog down the line.

 

Even with an OBD II car, there is no code that will ever tell you if a cat is actually "clogged" nor reveal any other sort of exhaust blockage. This is something that can only be determined upon physical inspection (either visual, or by IR temp gun, and/or by insertion of a test pipe). P0420/P0430 only indicates a reduction in oxygen storage capacity of the cat (this is what is meant by "efficiency below threshold"), which is a code you would also see if the cat was completely gutted (and therefore impossible to be clogged). With this in mind, disabling these codes does not lead to any increase in difficulty of potentially diagnosing a blocked/melted/dislodged substrate in the future - they only provide an alert that tailpipe emissions have crossed a certain threshold, but do not provide any information regarding exhaust flow. So, if you begin to have driveability issues that warrant a test for clogged cats, then having or not having P0420/P0430 active won't really help OR hurt the diagnostic process IMO.

With that said, I don't think that the cats are necessarily more likely to break apart or become blocked just because of the known condition relating to coatings wearing off the substrate; this just causes the converter to become inefficient at reducing tail pipe emissions - not inefficient in terms of flow. No physical blockage ever occurred with mine over many years and many thousands of miles after this code. FWIW, regarding the long term, I think a set of inexpensive aftermarket cats would probably be even more likely to have issues than your original factory cats (not to mention you might install them and still get a P0420/P0430).

Actually there is a cheaper option. Looks like they do still sell O2 sims ($120 + free shipping):

http://www.ws6project.com/user_stor/...or-simulators/

But, of course, the value of a basic mail order tune is that some other tweaks can be made for that $175 price, so it might be worth the extra cost if you want to get other things done.

 

What about the old spark plug non-fouler trick? That’s even cheaper but I’m not sure if it’d work. It’s been said to on other cars in the past, but basically it just doesn’t let the O2 thread in all the way so it’s not in the direct exhaust stream.

 

I've heard of this, but never tried it myself on an LS1 car (or any other car for that matter). But this would only potentially work if the problem is in fact the cat itself (which is likely based on the GM TSB, but I suppose there is a small chance that it could just be a fault with the sensor). If the issue happens to be a fault with the O2 sensor, then moving it out of the exhaust stream wouldn't make any difference. Of course, you could move the O2 sensor to the other bank and see if the P0420 becomes a P0430 - or, just try the anti-fouler trick and see if it works. But, O2 sims/tuning will definitely work regardless of whether it's the cat OR the O2 sensor.

 

It sounds like I have a vacuum leak but it’s not running rough. It whines like it supercharger with throttle input and it’s making the air sucking sound like a cold air intake. I’m using the OEM box. When I shut it off you can hear a whoosh of air. At first I thought it might be PCV related because I changed it as well as the hose that runs from the valve to the manifold. However it sounds like it’s coming from the drivers side more so. I sprayed around with tb cleaner (all I had) but hadn’t found yet it. The P0420 came back on today. Would/could a vacuum leak cause a 420? Drivers Side
Passenger Side
https://m.youtube.com/shorts/BQ6HPskrbvI

Engine Shut Down Whoosh
https://m.youtube.com/shorts/jg-EI6sBKbc

 

Passenger Side

 

I can't watch any of the videos, I'm currently PC-handicapped. But a significant vacuum leak would normally cause some idle issues, including IAC counts outside the normal range (such as near 0) in an attempt to control the idle. You can also look at fuel trims to further confirm (or rule out) a vacuum leak, they should be elevated (max of 25% LTFT) on the side with the leak (assuming the leak is side specific, such as driver's side/Bank 1/"B1"). I wonder if there might be an AIR system issue (such as a breach in the line between the lid and pump), or maybe a split in the bellow between the MAF and the throttle body (which would allow unmetered air to enter the system, and likely show elevated fuel trims on both banks, but not technically behave like a vacuum leak as it's in front of the TB).

 

I think the rubber couplers that go to the TB & MAF are good all the way around. It’s hard to get a good look at the bottom but I can’t feel any air moving and when I try to seal it with my hand it doesn’t seem to change sounds. What about the TB gasket? I had it off to clean it and I reused the old one. I can’t remember what kind of condition it was in. If the gasket were bad do you think the engine would be stumbling?

 

Well, a leak at the TB gasket should cause elevated fuel trims on both banks...have the fuel trim numbers changed since the earlier image that you attached?

Can your scanner show IAC (Idle Air Control) counts? If the idle speed is normal and stable, then the leak would have to be small enough to be within a range for which the IAC can compensate. But if the IAC counts are at or very near 0 (meaning fully closed), and the engine is still able to idle normally, then the engine is drawing significant air from an improper source.

 

Here's a recent thread you might want to check out, another member who is fighting the same problem just reported great luck with the defoulers and did a nice write-up on the process:

https://ls1tech.com/forums/generatio...p0430-7-a.html

 

I noticed that it had the slightest little hiccup when it went to “warm idle” so I decided to pull the IAC to have a look see. The o-ring fit like a pair of baggy pants so I went to the parts store and matched one up. I think what I got is an A/C o ring. For better or for worse I RTV’d the IAC where one might find a gasket if there was one. So far so good. It’s idling well and the CEL hasn’t came back on but I’ve probably only been 30 miles or so since I reset it.

 

if the four O2 sensors are working, give it a try…$7 and a couple of hours…worked for me, I am sooooo happy right now.
here is the thread…
https://ls1tech.com/forums/generatio...p0430-7-a.html

oops, I just noticed you already pointed to the thread…sorry for the double post and thank you for sharing it :-)
I sure hope this helps Y2K_Frenzy

 

I drove it probably another 15 miles or so today and still no check engine light. I think it’s been around 40 miles or so since I cleared it and it seems like it came back on sooner the last time but I don’t remember exactly. But I think maybe hopefully it was related to the IAC o ring. We shall see. It’s in the shop getting the rear end looked at now.