[A4] Catalytic Converter

Peter Kirby 4pkirby at gmail.com
Sat Nov 11 08:45:21 EST 2006

Yeah, the second O2 sensor is there to detect the operation of the cat 
-- it serves no purpose in monitoring fuel/ignition.  If the cat fails, 
the second sensor will trip the CEL.  However if the second O2 sensor 
fails the CEL light will be tripped as well.

Ask the dealer specifically which code was generated.

If anything, I would thing the cat would work worse when cold, not 
better.  Without knowing the specific code, my guess is still a failure 
of the O2 sensor, not the cat.  If the cat were plugged up you will feel 
an effect on performance.  If the cat were blown out you might here a 
rattling sound if you went underneath and thumped on the cat with your 
hand or a rubber mallet.


Kent McLean wrote:
> Ken Witherell wrote:
>>Thanks for the advice.  Dealership said they ran the
>>diagnostic and the codes turned up the catalytic
>>converter.  Dealership is Stammler in Boulder, CO. 
>>BTW, as soon as it has gotten cold here, the damn
>>light has gone back off?
> Someone correct me if I'm wrong, as I may be off base here...
> My understanding of the OBD-II diagnostics is that it reports
> about the sensors in the system, whether they are working and
> within range or not.  What sensor is there that would finger
> a catalytic converter as being bad? That is, what does the
> OBD-II diagnostics test to determine the cat is bad.

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