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Thanks Ray. Every reader in this group may have idea who is expert and
who always provide sound advices to newbies like myself. The learning
curve may be too deep for me and I try to ask more stupid questions as
possible as I can. Please bear with me.
Sorry I didn't address my question clearly. I have two Corollas. 2000
Corolla has new CAT and 2 O2 sensors replaced and ready for State
inspection(due to bad CAT). My other Corolla(2001) had check-light
on(MIL was set) in July and faulty code is P0420 which is the same one
that my 2000 Corolla had/repaired for, and I had it cleared in July.
But since then CAT monitor has been stuck for my 2001 Corolla and about
2 months have passed, still the monitor cycle is not done. So, I
followed your advice and subscribed to Toyota techinfo.com and obtained
the procedure to run the drive cycle. Now, the CAT monitor is done but
no faulty code. That is reason why I was asking you why check-light
went on at the first place in July. If check-light was on and faulty
code was P0420 in July, now it disappears, what does it mean for the
same car?


Ray O wrote:[color=blue]
> <wenmang@yahoo.com> wrote in message
> news:1158894593.269253.238880@d34g2000cwd.googlegroups.com...[color=green]
> > 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)[/color]