Hey all! I bought my beautiful '97 about a year ago, and I have been battling with the check engine light ever since. The code says P 401, EGR Gas Flow Insufficient Detected. We have checked the sensors and switches for the EGR, repaired the vacuum plumbing, and installed a missing check valve, removed the EGR, checked it, cleaned it out, reinstalled it, repaid the seal ring on the fill cap of the gas tank, and just had ALL FOUR of the Oxygen Sensors checked at Audi, and we've cleared the code after all of the aforementioned repairs. HELP! Thanks in advance.