Needed to replace the catalytic converters in my 1997 SL600. Took the car to the shop for this work and wanted to use the magnaflow 99005HM as the replacement units, that didn't work. the original cats on the car have four 02 sensors, the ones before the catalytic are screwed rigth on the cat converter, the ones aft, are screwed on the pipe connected to the cat. Any ideas or advise please. Thanks.