On Sep 26, 11:19 pm, "Ray O" <rokigawaATtristarassociatesDOTcom>
wrote:

There is a long history to my cat replacement, which was discussed in
this thread here:
http://groups.google.com/group/rec.autos.tech/browse_thread/thread/99a4e98aff0cf8a9

The summary of that issue was, I initially replaced it with a crappy
aftermarket cat. It was fine for ~6 weeks until I had got a fuel
injection service. A couple of weeks after that I got the P0420. There
was discussion on whether it was the cat that was bad, or the fuel
injection service damaged it, or both.

Ultimately, I took it to my trusted mechanic, (the guy who put the
crappy cat was a local guy, not my trusted mechanic) who replaced it
with a superior quality cat. If I recall, it was either an oem for
another car model or just superior in quality. Since then, I didn't
have any issues with that cat.
***************

Looking through that thread, there is a mix of good and bad information and
advice.

Aftermarket cats are a hit-or-miss proposition, and I've read and heard of
more people with bad luck than good luck with them.  As expensive as OEM
cats are, IMO, they are the only way to go.

At this point, check the O2 sensors as I mentioned above.  If the Bank 1
Sensor 2 checks out, then the cat isn't working properly.

On Sep 27, 1:51 pm, johngd...@hotmail.com wrote:

Since I just replaced the bank 1 sensor 1 with the OEM sensor, I
suppose it would be okay to "mix and match", and replace the
downstream sensor with this bosch one? After my cat experience, I'm
just too skeptical to use non-oem parts for emission components, so
pardon me if my questions sounds silly.

Thank you.
************
It is OK to mix OEM and aftermarket parts, but before you go and replace any
sensors, check their function.  I suspect that B1S1 (Bank 1 Sensor 1), B2S1,
and B1S2 are all OK because the nature of P0420 means that B1S2 sees a
signal that matches B1S1 instead of a more steady signal that would result
if the cat was working.  If the sensors are good and the cat is bad, you
would probably end up with the same trouble code.

Checking the sensor output voltages and frequencies would eliminate them as
the cause and point to a bad catalytic converter.  If you don't want to
bother checking the sensors and prefer to just replace parts in hopes of
solving the problem, replace the aftermarket cat with an OEM one.  Of
course, an OEM cat is fairly expensive, so spending 10 or 15 minutes to
check the sensors is probably worth the time.