Hi im new to this forum, I have a 1998 es300 Lexus that has about 67,000 miles on it, i got it in 2006 with 49,500 miles on it. Im the 3rd owner the first 2 were brothers i am friends with and im pretty sure it was a second car gone unused for either of them. i dont drive much and i dont drive it hard and ive had the oil changed on time during this time. this is my first issue with it. it was driving odd to me for a while and then cel come on and the code is 420..my dad says should rule out the easy things first and so we got new plugs, my x husband installed them only to find he put wrong kind in and then when he did install right ones he did the firing wrong and it sounded as if he broke the car, but after i looked up a diagram he redid this and it was ok again, least it started and ran, only it still felt funny. my dad realized he only changed the front plugs and changed out all of them finally and it seemed great for a while.. then the light came back and now it feels odd again. so i am wondering what is the next thing to rule out and is there any where for me to read how to do this or instructions? ive been driving like this for a few months.. and i dont work on cars so im just trying to learn because i want to be doing the right thing the right way in order to keep my car in best running condition. with what ive learned it seems to try the o2 sensors? or converter? is there a book for this exact car with part numbers and diagrams to fix any issues such as these? i dont think i feel so confident in my x working on this without my having knowledge of whats going on, i dont think its something he cares to do right or learn about but he has worked on cars in past and HAD a interest. yet as interest diminishes so do the skills it seems.. my dad has worked on my cars all my life but he is familiar with "older cars" not these "new cars" &distance has made that not possible as of now so i am trying to figure it out. id appreciate any help, ive been reading the forums for hours now! Thanks so much!!!

 

P0-420 is the catalyst efficiency below threshold code. It can be a few things.

The actual Air-Fuel Ratio Sensor is in need of replacing

-or-

There is a vacuum leak in one of the many rubber vacuum hoses

-or-

The Mass Air Flow sensor is in need of cleaning.

but i believe least likely is that the actual catalytic converter is bad. It is possible though.

hope this helps

 

yes it helps!!! Thanks so much! i am now armed with new information and can proceed trying to fix this with a lil more direction! I guess i assumed the hoses would have been checked already...i dont know what im doing so again thank you very much!

 

P420 - Catalyst Efficiency Below Threshold

Check for exhaust leaks, make sure car is in fuel control, bad post oxygen sensor, and lastly bad catalytic converter.

 

I get this code all the time at my previous work and I can tell you that you might be chasing demons....

Per the other posts...its a cat efficency code...which means that im pretty sure that the tune up wasnt done on time as recommended by toyota/lexus. I actually recommend doing a tune-up before the 60k, like around 50k....

Since the spark plugs were probably fouled out, it send the extra fuel to the o2 sensors, contaminating them and killing your cat.

Long story short, you will probably need to change your plugs, then need to change your o2 sensors( all) then most likely will need to change your cats.....

Good luck and keep us posted

 

Id put my money on a bad catalyst. It could also be an exhaust leak or a bad o2/air fuel sensor but in the greatest likelihood the catalyst has gone bad. You could try clearing the code and see if it returns, but you'll probably have to replace the catalyst.

 

I don't have a solution, but a follow up question on all the possible issues. I know that sensors can be tested, vacuum and exhaust leaks can be found, but how do you diagnose a bad catalyst? Do you just have to run an emissions test after ruling out all the other options?

 

I have been fighting the same issue. I checked/changed all of the cheaper fixes to no avail. My question is, if the code says catalyst efficiency bank 1. Which of the 3 cats do I change. The logical one would be bank one but There are 3 O2 sensors, one before each cat on manifolds and one after the main cat on exhaust. How does the computer know if it's cat on bank one? Also, want to confirm that bank one is the rear bank (closest to the firewall).

 

If one of the 2 Air Fuel sensors were bad, you would throw a "hard" fault on the CEL - it would come on instantly w/ no "pending" code in the ECU (meaning the ECU has to "see' the same fault twice before the CEL comes on.).

I have battled this P0420 code after changing out the exhaust pipe that holds the catylatic converter due to an exhaust leak in the flex pipe (was an aftermarket pipe, not OEM Toyota. Toyota wanted $1200, aftermarket was $500 - no brainer decision). Had my mechanic change it out and after 50 miles of driving, yup, CEL P0420 code.......Started the process of checking everything that could cause the CEL, checked Air Fuel sensors w/ a DMM (Ohm resistance), checked the O2 sensor after the CAT w/ DMM - All were fine. Checked for exhaust leaks - nothing........

After doing some intense research and talking to a few Toyota techs, the tolerances in the ECU are very small - there is very little "gray" area when the ECU "reads" the sensors. Therefore, the aftermarket pipe I put on which is 50 state legal, was causing the ECU to "see" the CAT as not working to its FULL potential......since there was nothing else wrong with the car, MAF sensor was cleaned and fully functional, no vacuum leaks anywhere, etc......Installed a reisistor on the O2 sensor to fake out the ECU thinking the CAT is working as it should.

To this day, I have not had any P0420 codes or any underlying issues with my car. It drives normal, gets great gas mileage (alomst 425 miles to a tank), and is reliable.

Someone else said it best - P0420 code is like chasing "ghosts" throughout the car. The code is too generic and if it is the only code that triggers the CEL - good luck finding the culprit. If you get other codes in addition to P0420, then you have a fighting chance.

 

You would need to isolate the problem one by one before ruling out the cat. I list the procedure on my previous post.

 

o my goodness.. overwhelmed!. so thankful i found this forum because i totally feel like i been chasing ghosts... and i have no clue what im doing! thank you everyone who has responded so quickly i really am appreciative of your helping me out! i am completely ignorant to what goes on under the hood. i only know noises and smells to be aware of in a 1982 mercury ln7. & that a loose A10 fuse causes a car to seem more messed up than i would think a loose fuse would..
so what ive done is checked vacuum lines and cleaned the maf sensor.. then cleared the code. the car still feels off to me , the check engine light has not come back but i will be floored if it doesn't, ive cleared it at least 5 times already & it hasn't failed to come back on!
then i sat down and read some more... and then started to look for a catalytic converter.. i find universal to be affordable at $60-70..and then they start shooting up from 250 to 450!! what in the world? x says i probably need to get 2 or 3 on my car?(hoping this is a example of our communication issues in this case) and 3 sensors? i thought a car had one catalytic converter... what parts should i be looking for to fix this?
i need more than the service manual to check this information specific down to every bolt and screw to my exact car & pictures of placements of everything. is there a specific book anyone recommends? im making this the next step before going back to the checklist ive been making.. it looks like a dysfunctional family tree at this point.
What kind of resistor did u use? i am having this code appear all alone, not any others have shown.. its been a few months & i wonder if driving it like this is making more damage & how long i even can drive like this before its dead or what will happen? again i must say thank you to everyone...

 

my dad said he seriously drives it 30mph and then punches it... he says he can tell if its bad this way everytime. though i assume this would be after checking all other problems or if the only issue was finding out if the converter was good or not.

 

Once you clear the codes, you have resetted all the monitors. After you complete the drive cycles, and the catalyst monitor completes, it will trigger the CEL on again if it sees the fault after 2 consecutive trips.

If you car is a California model, your car has 2 cats. 1 WU-TWC and 1 TWC. To determine if you got a bad cat, you can try my procedures above.

 

you most likely do have 3 catalytic converters, and two Air-Fuel ratio sensors (theres a difference, AFR sensors are almost 2X the price as well compared to O2 sensors)

as for replacing it, you can bring it to any muffler shop and have them weld in a new catalytic converter. it might not be OEM but it's why you're seeing prices from 60-70. bolt on replacements will run you 250-450

good luck.

 

No offense, but the 98 ES300 only has 2 cats. One warm up, and the other is TWC.