[ischmitz]

As others said, check if you have an O2 sensor installed in the cat bypass tube. Then check and post the model number of the DME under the drivers seat. You can tell whether it has been opened by carefully looking for pry marks around the metal tabs. If there are none it is safe to assume the DME is stock. If there is any sign of pry marks you would want to open the DME and check what chip is installed. And once you have it open post an image of the lower circuit board. Then we can quickly tell whether it is an O2 capable DME. You can get a correct chip for it if it has the O2 portion of the circuit.

To get a cat working you need a DME that has the O2 sensor portion of the electronics, a correct chip, and a good O2 sensor. This will make sure the DME keeps the A/F ratio in acceptable ranges for the cat to work.

Ingo