After bringing in my 540 for the "check engine light" that they first thought went on because of not securing the gas cap, and instead they had to UPDATE DME, its back on again. What is wrong. I know that people had problems like this and it was the cat. converters. When I take it back, what should i suggest. What did updating DME do? thanks in advance.

That question can only be answered by the person that made the decision to update the DME.

What they did is download a newer version of the software. How that affected your system depends on what version they updated from.

They did this for one of 2 reasons:

- some technical reference suggested this should be done (they can tell you the details if this was the case).
They may have been reacting to an error message found when they did a diagnosis.

- they did it on speculation that it might help.

In either case - take it back and explain you want the fault corrected.

Cheers
Jim Cash