Opel Omega B Caravan, 2.5L V6, 125 KW Built `96: CO value too high

I have a question for you: I was at the DEKRA last time because of the ASU with my Opel Omega 2.5L, V6 125KW, built in 1996. The ASU didn’t give because CO-value would have to be 0.3. My Omega unfortunately had 0.56….. is the verdict not to deliver me the ASU ok. or do they ask themselves a little petty? Also on my question, where could possibly be a source of error for it, they could not really help me. They only thought that it might be the cat or the Lambda system. In clear text this could mean: ellenlange Messerei, which u. circumstances could become really expensive or? I would be extremely grateful about every helpful tip from you. ….. By the way, did the car only have about 1.5 weeks. Greetings, Aki