Getting P0420 code: Catalyst System Efficiency Below Threshold (Bank 1).

I realize that it may likely be an exhaust leak, bad plugs, bad MAF sensor, or bad catalytic converter.

So far, I've tried Seafoaming the car to see if I can notice any smoke coming from the exhaust pipes anywhere near the O2 sensors, and did not see any smoke. I haven't cleared the code yet.

I was planning on replacing the spark plugs anyway, so will be doing that next, but I'm wondering about the MAF sensor...

Can a MAF sensor wear out over time to become less efficient, yet still continue to work well enough without triggering a Check Engine Light?

I ask because I will try to clean the MAFS as well, but even if it works well enough to not trigger the CEL, can it still run less efficiently than a new one? Or do they operate like coil packs? (either they work or they don't - no in between).

 

I don't think it's a MAF sensor related problem. If a MAF sensor was failed, you'd have a code P0730 too. You can check the MAF sensor measuring the output voltage as I just have posted here.

 

i would swap the downstream o2 sensors from each other and see if the code follows to the other side. if it does, you have a bad sensor. if it doesn't then look at other solutions.

of course yes check for exhaust leaks first! it wouldn't hurt to change the plugs.

 

Yours has 230K miles so if these are the original O2, they are probably due for replacement. You can swap sides to see if the code changes to the other side but my suggestion since you are under the car is to simply replace both upstream sensors. Rock Auto they are $38 each.

Do you have the smog test results from the transfer process? Post the readings if you have them. Some possible clues there.

 

This code is pretty much a bad driver's catalytic converter. I have it and I tried all other solutions above and nothing got rid of it so far

 

My 95 LS400, same codes, replace the cat and findly found out I had broken EGR tube back on engine. Replaced and no more headaches

 

Interesting. How can you tell it was cracked? Can you somehow identify a crack while still installed on the car?

Also, you replace the entire EGR tube yourself?

 

a crack sounds like an exhaust leak near the upper intake manifold. it's pretty common on 90-97 models to have broken EGR tubes.

i tried to replace the tube myself but got 90% of the way there but the upper bolts are a pain to get to without the right amount of extensions and swivel joints.

more:
https://www.clublexus.com/forums/ls-...-egr-pipe.html
https://www.clublexus.com/forums/ls-...pe-repair.html


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Ah okay, so I’d be checking for a swooshing sound similar to an exhaust leak or a vacuum leak which would get more pronounced under throttle, ya?

Is that the only way? No visual possible or a way to smoke it out?

Also, mine is a 2000… Does it occur on the later models as well?

 

98-2000s do not have EGR pipes

 

Question:

So, I installed double stacked defoulers on the downstream O2 sensors in hopes of (temporarily) readying the O2 sensors to pass smog (while I hunt down a deal for CARB compliant cats).

I’ve cleared the codes, and some monitors are taking longer to ready…

Is it even possible to ready these sensor monitors with the defoulers installed?

 

the foulers just move the sensor from the exhaust stream, but they are still connected to the car so theoretically they should still eventually ready the monitors.

i just went through this with my neighbor (albeit it was a camry but it will be similar). it takes a few drive cycles to get the monitors ready, at least two full cold/hot cycles. be aware of what you google as some instructions will vary by car model - unfortunately i don't know what it exactly takes to ready the monitors on an LS400.

some say it's based on mileage or holding a certain speed for a set amount of time, but from my experience with my neighbor and my own vehicles you just have to drive it for more than 20 mins to get the coolant/engine oil up to temp then back to cold, at least two times.

 

I completed driving patterns 3, 6, 8 and 10 from the TSB about it:

https://www.clublexus.com/forums/att...e-patterns.pdf

I spent like 5-6 hours today driving around doing these driving patterns, still not ready.

I’ll keep trying. I’m glad to hear that it is possible.

 

wow that is a pretty comprehensive TSB! thanks for sharing that!

when i did it, it spanned across 2 days since there needs to be a cool down period (starting from cold). essentially i drove to work the 1st day, then back home and repeated the next day and it was all ready. my one-way commute is about 50 mins mostly highway with 20 mins of city driving in between. i did not do any patterns, just drove normally.

 

I sure hope you can get your stuff straightened out. Even though this doesn't apply to my car (93) I read that TSB anyway. I can't even imagine doing all those driving patterns. Actually, in my city, there's nowhere to drive in those patterns. I'd have to drive an hour away to start. Even on the highways with all the traffic I don't think it would be possible. Good thing we don't have emission testing over 25 year old vehicles. There's so many reasons I like my 93. Good luck with this.