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<wenmang@yahoo.com> wrote in message
news:1158894593.269253.238880@d34g2000cwd.googlegroups.com...[color=blue]
> Ray - update:
> CAT monitor is finally done after I followed the instruction(for drive
> cycle) obtained from Toyota techinfo.com. The P0420 disappeared and
> didn't come back. Everyting is passed and no faulty code is stored. Why
> is that? and what happened at first time(P0420 appeared and I had it
> cancelled unaware that CAT is still under warranty)? Is CAT good or O2
> sensor is marginal? Currently my 2001 Corolla has 5K miles shy from 80K
> for CAT warranty, I may save money if the faulty code occurs before 80K
> miles. What is your suggestion?
>[/color]

Wenmang, I am flattered that you sure seem to have a lot of trust in my
advice. So you and the other readers can evaluate the quality of the advice
I give, I'll let you know that I derive most of my advice based on a
combination of remembering something I read at some time, from a class I
took about 20 years ago, experience, and a basic understanding of how cars,
electricity, and simple machines work. I try to look at a problem at its
most basic level, remembering that an engine needs air, fuel, and a source
of ignition at the appropriate time; that E = I * R; and simple machines are
the wheel, lever, and inclined plane. If you remember that basic stuff and
realize that just about the entire car is based on those principles, you can
derive the solution to most car questions.

The P0420 trouble code probably cleared itself because it is one of the
codes with 2-trip detection logic. If the same fault is present over 2
drive cycles, the ECM will turn on the MIL (malfunction indicator light -
check engine light). Conversely, if the fault is not present over 2 drive
cycles, the ECM will cancel the code and extinguish the MIL. After you
cleared the codes and went through the drive cycles to get the ECM into the
ready state, the ECM probably has not detected the fault over 2 drive
cycles.

If you did not change the catalytic converter or O2 sensors, my guess is
that the same fault will appear again, hopefully, while it is still covered
by the emissions warranty. If it does appear, do not clear the codes. If
the car is still within the emissions warranty, take the car to a Toyota
dealer for repair. The O2 sensors are probably not covered by the emissions
warranty and the dealer will probably want to replace them, but they are a
lot less money than the CAT so IMO, it is a good deal. If you had the CAT
replaced, then the fault is probably cured.
--

Ray O
(correct punctuation to reply)