P0 420 Code On 95 Lexus

[95LS400Bob]

After a highway cruise last night...we got a check engine light. So went to Autozone and read the code P0 420. Read the diagnostic in the manual and basically it means the 02 sensor before the cat on the left bank is not seeing the same "wave" at the 02 sensor after the cat.

So..those of you who are up on this....If I had a bad 02 I figure I would get an additonal code telling me rich/lean whatever. But I only have one code.

So, I figured maybe I better look at the cat. So I pulled the exhaust pipes off right at the cats...started and ran the motor. Exhaust felt equal coming out. So no plugged cat or anything.

Then I pulled the cat off the left side (takes forever as bolts are frozen...don't do this unless you want to spend the day) and looked through the left side cat...all looks fine..no broken pieces and everything clean.

So I reinstalled everything...reset the light. But it will probably come back on again after another cruise.

Anybody been through this? I know Bad 02's are possible...but wouldn't I get another code?

[95LS400Bob]

Seems nobody replied to this probem so I must be alone. I'll make this an information thread for others in the future.

The 02 sensors are a critical element in our cars. The signals they send tell the engine computer to richen or lean the fuel mixture for best power, economy and emisions.

These sensors live in constant heat and are subjected to possible exhaust contamination. It's my belief that they can get slower in sending the info to the PCM and hurt performance and economy. Yet they might not turn on the check engine lamp as long as they are working somewhat.

My reason for thinking this is a 1993 Cadillac I bought with 70,000 miles. The car ran perfect (so it seemed to me) and seemed perfectly normal. No check engine lights were on. I bought the car and when I went so have it smog tested it failed..but just barely. The car was freshly tuned up and had new oil in it when the smog test was done. I was kind of at a loss of what to do. So I went to an auto parts store and bought an 02 sensor. Even the guy behind the counter said he didn't think it could be that...for my car ran so perfectly.

I installed the new 02 and went for a ride. The car had lots more power than I ever thought a Cad would have. I thought for sure I had gotten the wrong 02 sensor..and this new 02 sensor was giving the car a lot more fuel and hense why it had so much more power. Surely it would fail the smog test with this 02..big time.

I knew the previous owner well and took the car over to him and had him drive it. He said the car never had that kind of power during the 30,000 miles he had driven it while owning it.

I took the car to a smog shop and the test passed with flying colors. Near 0 on emisions. A highway cruise gas mileage check came up 28 mpg.

So now I have this 95 Lexus. It also runs smooth and quiet. But did it used to have more power or get better mileage? I don't know as this is the first Lexus I have had. But the 93 Cadillac would blow it's doors off. But I still like the Lexus better.

02 sensors are expensive (like everything else) for these cars. So I don't want to just throw parts at it. I'm well connected in the auto community and a friend of mine and I are going to run some scan tests on the 02 sensors to see how good they are operating. My car has 150,000 miles and the 02's are all original.

I'll keep this thread posted with our finidngs.

[blake918]

I had a P0430 code in early August-about 233k miles at the time. I thought I was going to have to shell out big bucks for the cat. I read in the repair manual that a Heated O2 sensor could be at fault also. I was hoping that a piece of garbage or something was on a sensor, so I locked the car down in second gear and brought it up to about 75mph so that the engine was turning many, many rpms. Sure enough, the next day, the check engine light went off. :D I was kind of sad to see the cel go because while it was on, I didn't get anything under 26mpg.

I look foward to hearing how the new O2's effect your car! I wouldn't be suprised if mine are origional either.

[95LS400Bob]

Just got back from doing a complete scan on the Lexus. We also put it on a lift so i could get a better look standing up..instead of lying on my back.

Found that fuel filter...and it is original and never a wrench on it. The rear suspension and the bushings are in good shape so no further suspension work is needed.

The rear 02's read near same values as the car is running. Switching rapidly as they are supposed to. That pretty much ruled out the rear 02's.

But the pre cat 02's..the ones that control the mixture...there is quite a difference between banks. From all indications of their switching patterns from rich to lean (which is what they are supposed to do) it is obvious to us that bank 1 is leaner than bank 2. (Bank 1 is cylinder #1 side..driver side). And every once in a while..we see a rather "wild" number the left 02 give out. But reading the "trim" (fuel system being trimmed by the ecm) the trim looks pretty good.

Since my check engine code pointed at the left side (bank1) which shows leaner than bank 2, our first thought was to swap 02's from left to right side...and see what that would do.

But while running the scanner...we would hear a slight ping from the motor...especially on starting it. This meant the fuel is not enough octane. So we are going to get a new tank of fuel before we do anything.

Next step is going to be to drive the car over the same ground I did before when I got the check engine code P0 420. See if the code comes back with different fuel. If it does..then I am going to switch 02's right to left. And drive the same ground again.

We figure if the 02 is a little bad on bank 1 and causing the code P0 420, then it shoud do the same thing on Bank 2 if we swap the 02's and give us a P0 430.

[99lsguy]

95LS,

excellent diagnostic!!!

I'm staying tuned!! :D

[95LS400Bob]

Since there is an 02 thread going on, I thought I would update this thread. I've since bought a scanner system and put it on my laptop. The trouble code has never appeared again since we changed to a different fuel. Seems the wife thought mid grade was good enough and while the car did not ping on mid grade, the computer was definately dialing the motor back on power.

I confirmed this with the scanner which also does readings while driving. The computer was taking timing out of the motor on the poorer gas...as it was designed to do.

I read the wave patterns of the 02's both pre and post cat and the waves from rich to lean look as they should.

Fuel mileage also increased on the premium fuel.

We have a trip to Phoenix coming up in a couple weeks, I'll be armed with wrenches and the scanner and should the code appear again I will swap the 02's from right to left. I'll have fuel mileage readings when I get back.

[srgilbert]

Perhaps I have the very same problem. The check engine light went on and I have a 136 and a 420 code on Bank 1. The 420 points to a weak converter....yikes. I was thinking about replacing both fronts - my Lexus mechanic friend concurs before something more dramatic. I have been running 93 octane for 2 years (as long as I have had my 95. Could this be a bad tank of gas? Before I plunk down about $100 for 2 sensors and some time on the ground I'd appreciate thoughts on the best way to isolate bad gas from a bad sensor.

Thanks!

[92Lex]

Get a digital storage oscilloscope and hook it up to your O2 sensors....take a look at the cross counts on the scope and make sure it's switching from rich to lean evenly. It should cross the .450mv mark at least 2 times per second....also if the voltage dips into the negative value then you have a bad sensor. Make sure you test and check other sensors first because based on the order of influence the O2 sensor isn't the most important one. Order of air/fuel influence is : tach, maf, ect, iat, o2.

[srgilbert]

Wow - a scope is beyond me - any other suggestions for a backyard mechanic?

[92Lex]

If you think it might be the fuel you're using that's at fault then switch to a different station and see how it goes.

Or switch your O2 sensors to the other side....if you get a code for bank 2 then there's your problem.

[99lsguy]

So are these o2 sensors about $50 each........?

I'm not saying that's money to burn but---------------$100 to fix a part(s) that's 7+ years old would be no biggie for me..........

[srgilbert]

I just ordered 2 Denso OEM ox sensors off of Ebay $97 delivered - you're correct this is a wear item so I should just change them and move on. Does anyone have a good procedure to remove the sensors? Can I use an open wrench? DO I have to worry about these being frozen in or breaking?

Thanks again!

[92Lex]

I just ordered 2 Denso OEM ox sensors off of Ebay $97 delivered - you're correct this is a wear item so I should just change them and move on.  Does anyone have a good procedure to remove the sensors?  Can I use an open wrench?  DO I have to worry about these being frozen in or breaking?

Thanks again!

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Go to www.lexls.com , he has a O2 sensor tutorial with pictures if that's what you're looking for. Open box wrenches are fine...but if you have access to a flare nut wrench or O2 sensor socket it would be even better. Hard to remove....yes....but it shouldn't break on you (it's like removing a spark plug)

[Jacob]

srgilbert,

I changed my O2 sensors a few months ago, and I had to use the special oxygen sensor wrench which I got from Autozone (Loan-A-Tools). Good luck.

Jacob

[srgilbert]

I had a P0136 Bank 1 Sensor 2 failure and replaced the sensor. Should the check engine light go out on its own or will I need to pull one of the fuses to reset it? If I'm way off on this let me know. I also had a P0420 which is a weak cat so this may be the problem??? I have 105K on my 95 LS.

Thanks

[92Lex]

Do you still have a P0420 (bank 1 low threshold) code? Just clear the codes and see if you have a hard fault (if they reappear) or not. After repairs you should always clear codes just to make sure your repair was successful. If you're able to review the freeze frame data it would be easier to get it to turn itself off. The best way is to have the computer to turn the CEL off itself....but the easiest way would be for you to do it manually.

[srgilbert]

Can you share the easiest way to clear codes on a 1995 LS?

Thanks

[92Lex]

If you go to Autozone they should hook up a scan tool for you and clear the trouble code....this is probally the easiest way. You could also borrow a OBD2 scan tool and clear the code yourself...Autozone also lends them out. Or you could just disconnect the battery cables (negative first) for 10 minutes...this just turns off the CEL so you'll probally still have the code present....if it's a hard fault then the light will reappear.

[95LS400Bob]

Back from my Phoenix trip. No check engine light all the way down there. But right after I got a tank of fuel in Phoenix...the P0420 code has returned. The plan was that if it returned on the trip..I was going to swap out the rear 02's, reset the check engine light and go again. But it was raining and it never let up till almost home. I did clear the code with my scanner in Vegas...and now the code has set again. Perhaps now I will find it. Few other issues with the car also...but I will start another thread for that.

Still thinking this may be fuel related. It's taken a long time to get that code to set again. I'll be posting back as to what I find.

PS: 26.6 mpg gallon average. Mostly 75 mph. A few 100 mph blasts. No oil consumption.

[92Lex]

Back from my Phoenix trip. No check engine light all the way down there. But right after I got a tank of fuel in Phoenix...the P0420 code has returned. The plan was that if it returned on the trip..I was going to swap out the rear 02's, reset the check engine light and go again. But it was raining and it never let up till almost home. I did clear the code with my scanner in Vegas...and now the code has set again. Perhaps now I will find it. Few other issues with the car also...but I will start another thread for that.

Still thinking this may be fuel related. It's taken a long time to get that code to set again.  I'll be posting back as to what I find.

PS: 26.6 mpg gallon average. Mostly 75 mph. A few 100 mph blasts. No oil consumption.

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Is that the only code that is present? Fuel related? How so? Like low or high fuel pressure or do you mean a bad tank of fuel?

[srgilbert]

I have replaced both front oxygen sensors and the P0420 code continues to come back on my 95 LS400 with 104K. I have erased this code multiple times but after 10 miles or so the check engine light comes back on. I understand it is rare to have the rear oxygen sensors fail and am surprised that the cat (bank 1) would go out at this mileage.

Does anyone have thoughts as to how to isolate this problem? I hate to throw a new cat on it.

[95LS400Bob]

I would swap the rear 02 sensors. If the code sets up again..then I would have to say the cat might be the problem. But if you get a right bank code(P0 430)..then that would prove the rear 02 was bad. That is how I would do it before buying a cat.

I haven't been getting the P0 420 code anymore. But that is my plan if it happens again. I have pulled both my cats off and looked at them and they are not broken inside and look fine.

[95LS400Bob]

Back from my Phoenix trip. No check engine light all the way down there. But right after I got a tank of fuel in Phoenix...the P0420 code has returned. The plan was that if it returned on the trip..I was going to swap out the rear 02's, reset the check engine light and go again. But it was raining and it never let up till almost home. I did clear the code with my scanner in Vegas...and now the code has set again. Perhaps now I will find it. Few other issues with the car also...but I will start another thread for that.

Still thinking this may be fuel related. It's taken a long time to get that code to set again.  I'll be posting back as to what I find.

PS: 26.6 mpg gallon average. Mostly 75 mph. A few 100 mph blasts. No oil consumption.

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Is that the only code that is present? Fuel related? How so? Like low or high fuel pressure or do you mean a bad tank of fuel?

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Sorry I am so long in answering you. Must have missed your post. Not enough octane. The wife was using mid grade and we have not seen the code after changing to premium.

[srgilbert]

Excellent idea - will do - thanks!

I would swap the rear 02 sensors. If the code sets up again..then I would have to say the cat might be the problem. But if you get a right bank code(P0 430)..then that would prove the rear 02 was bad. That is how I would do it before buying a cat.

I haven't been getting the P0 420 code anymore. But that is my plan if it happens again. I have pulled both my cats off and looked at them and they are not broken inside and look fine.

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[srgilbert]

An update on my saga - I continue to get a P0420 code weak bank 1 cat on my 95LS400. I've replaced the front oxygen sensors with Densos and the problem did not go away. I took the car on a 300 mile trip last week after the ECM reset and the light came back on after 250 miles. I reset the ECM once again and I managed to get another 300 miles antil I hit town driving and the light came back on again . Today I planned to flip the rear sensors as suggested to see if the code goes to P0430 which will point to the other cat however I could not figur out how the rear oxygen sensors are fed through the frame - they are held in place with grommets to the body and I cannot seam to trace the wire after that to find the connector. I checked with Lexus and the cat is roughly $900 which is not an option. I'm finding that used ones do not seem to be available through junk yards due to EPA ....

Any thoughts? ANyone with the same experience?

THANKS!

I would swap the rear 02 sensors. If the code sets up again..then I would have to say the cat might be the problem. But if you get a right bank code(P0 430)..then that would prove the rear 02 was bad. That is how I would do it before buying a cat.

I haven't been getting the P0 420 code anymore. But that is my plan if it happens again. I have pulled both my cats off and looked at them and they are not broken inside and look fine.

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