There is only one sensor for the temperature sensor and temp gauge. The
previous model pre 95(B4 platform had two sensors)

It depends how long you drove around with a bad sensor. I wouldn't expect
the CAT to fail quickly. I don't know if your car displayed a CEL but it may
have not done so because it didn't finish the "warm up cycle".

Hi,

I've been told that the Catalytic converter on my Audi A4 Avant (1999)
1.8 Auto has failed because of the temperature sensor failing. The cat
failure must be bad, becuse the engine will only get the car to 30-40
MPH! I'm not convinced that it is the cat.

I was always told that the engine management system had a seperate
engine temp monitor. The sensor that has failed is the sensor for the
temp gauge on the instrument panel.

I've been told that because of the sensor failure, the engine has been
running permanently rich, which in turn has killed the cat.

Does this make sense?

Thanks
Dave