Hi! Guys

I'm from Bangkok, Thailand and need your help on my A8'97 ABZ engine. I bought this car fews days ago. My trouble is that CAT light came on once I started the engine. I went to the Audi specialist and use VAG to check this fault. It does not pick up any fault code on this problem. The car was de-cat and replaced with pipe. The engine is still run smoothly, I have no clues on this problem. Any one has ideas?