P0135 and now P0420 CEL

I have a '97 Lexus ES300 with 112K miles. I had got a CEL with P0135
code. I replaced the bank 1 sensor 1 O2 sensor with a OEM sensor. Two
days later, I get another CEL and now the code is P0420 - catalyst
efficiency below threshold.

I had the P0420 code about 6 months ago and I had replaced the cat.
Since then P0420 code was gone and everything was fine.

Any ideas why I am getting P0420 now, after P0135 and after I replaced
the O2 sensor? What should I be replacing now?

Thanks,
Mark

You should be diagnosing it with an OBDII tester. You can graph the sensors
and check their condition. Maybe the mew sensor is bad or installed wrong.
Did you use something to seal the wires where they enter the sensor? The
computer compares the primary and secondary to check the Cat efficiency. If
they are switching at the same rate you get the P0420. Good reading
http://www.troublecodes.net/articles/catfailure/


"Mark" > wrote in message
...
>I have a '97 Lexus ES300 with 112K miles. I had got a CEL with P0135
> code. I replaced the bank 1 sensor 1 O2 sensor with a OEM sensor. Two
> days later, I get another CEL and now the code is P0420 - catalyst
> efficiency below threshold.
>
> I had the P0420 code about 6 months ago and I had replaced the cat.
> Since then P0420 code was gone and everything was fine.
>
> Any ideas why I am getting P0420 now, after P0135 and after I replaced
> the O2 sensor? What should I be replacing now?
>
> Thanks,
> Mark

I took the car to my mechanic to have him check the other two O2
sensors. He asked me to buy 'catalytic converter cleaner' and put it
in the gas tank. That should clean the cat and the CEL should go away.
Anybody have any prior experience with this cat cleaner? Does it work?
Does it do any harm? I wouldn't mind trying it as long as it does not
cause more damage.