Hi,

I recently replaced the catalytic converter because the check engine light
would come on with P420 (catalyst efficiency below threshold (bank 1)). I
watched the O2 sensors with some software and they had similar outputs which
made me feel its the converter. After the change the back one is very
different than the front one so it seems that the new converter is working
much better than the old one.

Problem is that after the car warms up, when sitting at a stop sign pressing
the break there is a very bad vibration which I interpret as a rough idle.
If I switch to neutral or park, it goes away. It also seems to happen when I
go very slowly (almost without pushing the gas pedal) in reverse. Other than
that the car seems to run fine.

The car ran fine before I changed the converter.... The new converter is a
that Catco. My car is a '96 Toyota tercel with 105K miles on it.

Does anyone know why this started happening?

Thanks much!
Mihaela