Tim C. wrote:

> I just paid big money to have my cat. converter (and front 02 sensor)
> replaced yesterday. The dealership didn't give me a satisfactory
> reason as to why it would fail so early (at 80k miles.)

Tim,

The catalytic converter consists of small amounts of precious metal deposited on a ceramic substrate. The failure mode is chemical ie poisoning the precious metal with contminants like lead in gas or mechanical, breaking up the ceramic substrate. Plugging is mechanical failure (the ceramic substrate gets broken up into small fine pieces) and everything else being equal is usually caused by one of a few things.

First, normal thermal cycling will eventually cause the ceramic to start to crumble. Cats are designed to last 100k miles minimum but if the car is driven only on short trips (every trip thermal cycles the cat) then early failure could result.

Second, thermal shock will cause the ceramic to break up. Having the car (and cat) up to operating temperature and driving thru deep water for a long enough period will cause a thermal shock that could cause problems. This happened to me with about 80k on my 94 855T. In my case it didn't result in plugging, just large pieces rattling around.

In older cars a third failure mode could be prolonged running in an over rich condition where the cat gets over worked and over heated. This would not be possible in an 850 without tripping out other fault codes in the engine management system and lighting your check engine light.

Then again maybe it was just one of those statistcal outliers that fails early ;^/

Bill Matthews
Hockessin DE
94 855T
95 854GLT
73 1800ES