spuds

OK,1998 LS400.A bit back I replaced bank 2,sensor 1 (passenger side,upstream,USA) with Denso 234-4169,38 dollars from Walmart.com with tax.

Now I have P0420,Bank 1.....'Catalyst System Efficiency Below Threshold (Bank 1)' . So that would be the downstream sensor drivers side,is that correct?Part # Denso 234-4137,about 50 dollars at Walmart.com.

May as well get them all done as I now have 2 failed in a year,they are done for.

So.....I need 1 of the 234-4169 for the upstream I havent done yet,and 2 of the 234-4137 for both downstreams?Will cost 134 USD with tax and shipping,think thats about as good as it gets,eh?

I reset the code and if it comes back up I will order those three sensors. Sound correct? I used this thread from Scraape for numbers,THANK YOU!

https://www.clublexus.com/forums/ls-...em-and-oe.html

So,Im correct in all this,both the upstreams are the same, and both of the downstreams are the same? And I was correct on diagnosis as all the code shows is 'Catalyst System Efficiency Below Threshold (Bank 1)',its the downstream sensor that reads for Catalyst System Efficiency?

Thank you for your sage advice as always,much appreciated!

Sherl

I have the similar codes but did you try replacing the gasket, sealing the leaks and preferably sealing the top of gasket as well?

dlcoffin

yes, both upstreams are the same, and similarly, both downstreams also. if one assumes that both the upstream and downstream sensors both wear equally, are of similar age, then the possibility exists that replacing just one without the other can cause some disparity in readings enough to trigger the ecu codes. the fuel mixture is regulated by the upstream sensors by the ecu, and the downstream ones act as a sort of 'verify' sensor..someone correct me if i'm wrong and it's actually the other way around, please. it can also be truly the catalytic converter plugging up or failing enough to trigger the ecu for a code. from what i can tell, the p0420 refers in general to a detection that the catalytic converter is sub-par. doesn't really say if it's specifically related to the upstream or downstream sensors. i would replace the pair of sensors, and clear the codes, and run for a while, see if that code comes back. if not, good. it is the cheaper repair, as the catalytic converter itself and the repair/installation is hideously expensive in comparison.

spuds

Not sure what you mean,I have no leaks?

spuds

Thanks DL,most helpful.

When I replaced upstream a few months back,on Bank 2, car immediately came back with a p0420 (new code) after sensor change, on Bank 2 at that time.I cleared code,and ran car as advised,breezed thru smog check and it did not re-appear,until this one on Bank 1.Ive had no further issues on Bank 2 since,no codes anywhere in fact,runs great.

So OK,if code reappears I will get the 3 new sensors in,then all will be new(er),1 will be a few months old.When I did first repair really wasnt up to doing them all, as is frequently advised (good advice I see).After being under car and doing job,Im confident the rest will be not much of an issue to change.First one was simple,I spent more time pulling out tools and jacking up car than I did on repair.

Car has 249,xxx miles BTW,IIRC.

Yes,for sure Im doing sensors before cats.It needs new sensors either way

jaaa

Those codes come up when you have air leaks on the flange just past the catalytic converters. 420 would be on the drivers side cat and a 430 would be the passenger side cat. They usually corrode on the upper part of that flange and leak air. If you put your car up on jack stands with the car just started cold, you can feel and hear the air leaking past the flanges. If you find a leak, the sensors are probably good and the code is from missing air.

spuds

Thanks Jaaa,nope,no leaks,everything looked real good,no corrosion noted at all.Car is surprisingly clean.I'll be driving today and see if code pops back up.I'll update when I have more info either way.

I like those Denso parts,hope they dont get too chinese as discussed in some other threads.

oldskewel

iTrader: (1)

Knowing how accurate diagnosis is often the most challenging part of efficient maintenance, ...

If I ever got a code like that, pointing to a single cat or single O2 sensor being bad, I would have just swapped the sensor with its twin. Reset codes, and see what happens. If a code comes back, now on bank 1 vs. bank 2, you will have confirmed the sensor as being bad. If a code comes back, still on bank 2, you will have confirmed sensors probably good, and something else bad.

Yes, a little extra work that way, but if replacing the initial suspect does not fix it completely and permanently, it could become a puzzle.

At your high mileage, if you believe your sensors to all be original, yes, not a bad idea to just replace them all. But then you'll have to hope that it was a sensor problem in the first place, vs. some of the other options.

spuds

Well....Blackbird is just a good ole bird.

Temp 90F.Drove down mtn,freeway 20 miles,around town and up mtn.Not only did code not re-appear,all the readiness codes reset except for evap,which takes a couple sessions.Ran like a charm,A/C nice and cold,halfway on temp gauge going up the mtn.

So for now I can relax,and see what he does.And when it does come back,I know where to head for the fix.

Thank you ALL as always,really appreciated.

I love this car!

kambam3627

Spuds, I was recently having similar issues. What I ended up doing was putting a spark plug defaulter on the downstream sensor and the light went away for the po420. I’ve gone 700 miles and still no code return. Cheap thing u might wanna try.

spuds

Thanks,I will keep that in mind if this becomes an issue with smog down the line.