EthanS

Now before you all blow me up for another P0420 thread, just hear me out on where I'm at.
1999 LS400, 171k miles. I am second owner, received all dealership service records from original owner when purchased at 118k. Overall the thing is in great shape and has never let me down. Previous owner was very thorough and I am probably more so.
Only mods: I am lowered a fair amount on coilovers, but not "slammed". Hardly scrape on anything and when I do it's normally underneath the front bumper anyways.

About 6 months ago (literally 3 days after getting 2-year emissions and tags thank God) CEL pops on. I go get it pulled and its a P0420 bank 1 cat efficiency. Well, I just did emissions so no rush, I'll take my time trouble shooting and get around to fixing it, I've got another daily driver anyways.
The light/code continues to come on, and go away, come back, and go away, for the 6 months.. I cannot make any correlation between the light/code and driving habit, climate, fuel, or anything else; it is very intermittent. I CAN say that during periods when the light is steady on, the car absolutely DOGS in traffic, loses all of its ***** AND the mpg's go down the drain. I struggle to get 16 when normally I get about 20mpg..
I did notice too that when I really open the 1uz up, 6000rpm WOT, I have managed to "blow-out" the CEL on more than one occasion. It literally goes away right when I do a hard pull through a few gears... Hmmm
Other than loss in power and mpg the drivability remains smooth. The idle is slightly low (5-600), but it's always been this way.

Now that I've gained some motivation, I crawl under on Saturday and take a look. I check for leaks in the gaskets, in the welds, spraying soapy water on and feeling for air pulses around the bank 1 cat and exhaust piping with the engine running, and I can't find anything..
A co-worker recommends switching the post-cat o2's and see if the code switches from P0420 bank1 to P0430 bank2. I do so, pull the battery overnight to clear ECU, and still get the P0420 the next day.
It seems almost common knowledge around here that the cats should not go bad in a 171k mile 99 LS... If I have ruled out a bad o2, believe I have ruled out an exhaust leak, what else can it be?
I often read people saying "find out what other fueling or timing issue you have that caused the cat to go bad". What are these other issues? Bad plugs? Vacuum leaks?

Any help or suggestions is hugely appreciated!

timmy0tool

iTrader: (7)

are you properly clearing the codes using a reader (which I assume you are since you know what codes are stored)? pulling the battery may not be enough.
good job trying to narrow down the problem. next I would try switching the pre-cat O2 sensors. I wouldn't rule out the cats but always do the simple stuff first.
also, when was the last time you replaced your spark plugs?

Banshee365

I'm right there with you actually, on my '96. I had 420 and 430 codes in my LS400 before the one I have now. I replaced all of the gaskets and hardware on the cats with OE parts. I had an early software ECM causing intermittent stalling on highway off ramps so I installed a Lexus refurbed ECM with the latest software that someone had a dealer install in the car in 2012. The new ECM fixed the stalling issue but the 420 and 430 code activity stayed. I then got hit in that car from a speeding old man and had to build another LS400. I used the drivetrain from my first car into the car I have now. The 420 and 430 codes came right along with the drivetrain. The downstream sensors stayed with each respect car so different sets of those made no difference. I also swapped upstreams from the bad engine that WAS in the car I driven now. I also have no leaking at the cats. At this point I have to assume the mixture is running too rich or something causing the downstreams to still see a significant wave form after the cats to set the codes. Since I had an extra set of cats and mid-pipe I swapped those as well. Even though the cats in the first set looked squeaky clean inside I wanted to try it anyway. The codes seemed to go away for a week or so then just the 420 started up again and has been intermittent ever since.

I'm just about to give in and put spark plug anti-foulers on the downstreams and be done with it but I would much rather look more into the actual cause when I have to time to do so.

EthanS

You know what, I have just been having the Autozone on the corner pull the codes for me and I bet they haven't properly cleared them. I even had one of their employees tell me that they weren't allowed to clear codes??

I will plan on taking a crack at pre-cat o2 sensors this weekend, but can a pre-cat o2 even cause a 0420 or 0430? I was thinking that pre-cat sensors would have no effect.
I certainly have not ruled out the cat, in fact that seems likely that it is the culprit.
I have changed the plugs once since ownership. I'd say about 3 years and 25-35k miiles ago. I can probably find more specifics on what plugs I used when I get home tonight. I will update you on that...


I am getting there as well. Especially as my LS is my 3rd vehicle and mostly used for weekend cruising/meets and date-nights, one thing I will NOT be doing is paying for a new oem cat.
It just drives me nuts that when the light is on it has a noticeable loss in power and gas mileage. Hopefully the anti-foulers don't make that any worse?

aptoslexus

Banshee365

Anti-foulers will not effect how the engine runs. Being that you have low fuel economy and power symptoms, I bet your 420/430 codes are secondary faults caused by whatever is messing up your fuel trims.

With my car I feel I get slightly less MPG than I should but it's not horrid. I get about 19 mph with mixed driving.

Losiracer2

I had those P0420 and P0430 codes as well on my 97 LS. I just added 2 stacked spark plug antifoulers to the rear Post Cat o2 sensors to trick the ECU that the rear o2 sensors were registering less oxygen content. I drilled out the second one to accept the o2 sensor and then screwed that one into the first one, then screwed them both into the Y pipe (stacked with the O2 Like this < - < -o2) and I haven't received a CEL code yet.

Also, if you have any friends living in Maricopa or Pinal county, they don't do emissions there. Just use their address and then use your address for the mailing address. Then you can do 5 year registrations. I did that for my 93 Accord that had a Prelude engine swap when I was living in AZ to avoid paying emissions every 2 years. Makes no sense since Maricopa is just 15 miles south of the valley. I was hoping to get a copper historic AZ plate but my car is still not eligible.

EthanS

aptoslexus, thanks! Those screenshots are very helpful! Would you mind providing me with page DI-50 and DI-56 as referenced in the diagnostic flow chart at the end?

Thanks dude! I know Gila County and I believe Pinal County do not require emissions testing, but Maricopa County absolutely does. I've been living in Maricopa County my entire life. I did use this method on my sr20 swapped 240sx for years until it turned 21 years old and I insured and registered it as a collector car!

I will keep the county method and spark plug anti fouler ideas in my back pocket, not quite there yet!!

PS I love seeing your signature saying 28x,xxx miles! Gives me a warm fuzzy feeling that even though it has its annoying little hiccups, my LS will be around for quite some time!

Losiracer2

Oh yeah my bad, I meant the town of Maricopa, not the county. I forgot that there is also Maricopa county when typing that, probably shouldn't have mixed up towns and counties when trying to explain things.

Yeah 283k now, an AZ car it's entire life. I'm now in Michigan, so I'm worried about the upcoming winter and the havoc it'll wreak on the underside and rust. I'll probably clean it every couple days to proactively stop any of that from ruining this car and it's rather well kept body and paint.

aptoslexus

aptoslexus

EthanS

Thank you gentlemen.Will keep you updated!