bornking

Quick question
Got a P0420 error Bank 1
Replaced Cat on Bank 1
Code came back

However:
- Old gasket from the cat was worn and I didn't have new one.
- Didn't replace O2 sensor (downstream)

Question:
Could it be the gasket or o2 sensor?
or did I get a bad Cat?

jaaa

More than likely it's a leak on the rear flange where the cat meets the y pipe. If you put your car up on jacks and start the car from a cold condition you will feel and hear a breeze on the upper side of the flange. Doesn't do any good to replace the gasket, the part of the y pipe is worn away from long periods of gas escaping there. The hole just gets bigger and bigger. I patched mine with versachem muffler weld. It is basically a liquid steel product that hardens up after you apply it. I'm guessing your original cat was just fine.

Yamae

I'd check the output voltage of the O2 sensor. If it stays less than 0.3V before and after the warming up, I will check the air leak. The gasket or some other leaking point may be causing the problem.

bornking

I changed the y-pipes about a year ago with new ones...maybe a leak somewhere?
I will check!

billydpowe

iTrader: (1)

yep, go with jaaa

timmy0tool

iTrader: (7)

cats rarely go bad so I would have replaced the downstream sensor first before tackling a cat replacement.
http://www.easterncatalytic.com/educ...he-po42o-code/

RA40

iTrader: (6)

With the plumbing replaced verify that flange gasket. If the O2 sensors are original, test/replace. +1 with timmy on cat replacement. Have heard enough instances where car owners get into an expensive cat replacement to find that it still throws code.

Losiracer2

O2 sensors I think should be treated as a wear item. They won't last forever, and do get tired/worn-out over time.

1on1

My buddy had this same issue and I told him to buy the 02 sensor spacer/extender. This fixed the code.

RA40

iTrader: (6)

Some additional thoughts are that if the O2 sensors are detecting the output erroneously due to age then the air fuel ratio will not be correct. We'd look for other symptoms of these like poor gas mileage and reduced performance. On a car of these ages, O2 sensors are wear items so at 100-125K, they would be best changed out along with the coolant temp sensor, PCV for good measure. These are all low cost items to help insure that the ECU is properly regulating function. The reason being that something upstream is the cause of these "cat" failures. A rich ratio is a factor in seeing a non-functioning cat. For CA peeps when the inspector sees a spacer this would be a visual fail.

billydpowe

iTrader: (1)

YES !! the spacers are not a fix, just hiding the problem from your ECM, to pass testing.. fix it and make it run like it was designed to do.... BEST for you.

bornking

Update
Grabbed an obdII reader on amazon
It cycles between P0420 and P0430 leading me to believe it's the oxygen sensor after all...
Thy will be fixed!

blkstealthpaid4

I have this same problem on my 95.Which sensors do you think are bad?I hate to have to go inside my carpets to replace the rear sensors

RA40

iTrader: (6)

Lexus recommended is 100K miles on the O2, do both banks. Amazon has the factory plug connector Densos, on-off sale so you can get decent deals for them at times. I got $10 off on the bank 1 but the bank 2's was regular price.

bornking

Definitely bank 1. I have to go under the carpet...apparently the accelerator pedal can get in the way or something?