I have a 2001 Millenia with over 140,000 miles that is not long for my world. My fuel mileage has been dropping since the 70,000 mile mark and the dealer was supposed to replace the converters under EPA warranty even though the car had 101,000 miles. Don't think it happened and for a lot of reasons, I don't think I can get them to fix it. Of course replacing the converters is over $900 in parts.

I have some other problems and am looking at a lot of money to bring the car back to really good shape, so I plan on keeping the car at most for 10-15,000 more miles and many 6 months. I am down to 20 MPG and could probably run the defective converters until that time without much damage. However -

I know it is not legal, but I am considering removing the 2 front converters and just cleaning out the honeycomb and running them as straight pipes. I did this on a 1980 GLC Sport - but that was way before O2 sensors, computers, etc. And on this car there is a OS2 sensor in each pipe just after the converter.

So - what would I do to the engine computer and it running of my engine without the converters. In theory, I would be back to 27 MPG highway by removing the excess back pressure. But the engine computer would see the O2 problem and may make some adjustment.
I would not think it would be any worst that it was now.