P0411 code continues to come on after repairs to all parts affected by that code

Hi, I own a 2000 VW Jetta GLS 1.8T automatic transmission. The car has about 85000 miles on it. Last year, the check engine light came on. I brought it to my mechanic, he checked it with an OBD scanner and code p0411 popped up. He changed the air pump and the code went away. Less than a week later, the light came back on, same code so he changed the o2 sensors. Same thing happened. He also changed the combi valve, checked the hoses for leaks (which there were none), and also checked the vacuum hoses. One was collapsed and he replaced it. His last resort was to update the computer software, which he finally did. The light has been on for over a year now and my regular mechanic cannot figure out what is wrong with my car. I bought a new gas cap for the fuel tank thinking it was that since the very first time it came on was after I fueled up. Any suggestions?