have a 2002 tt quattro (no mod.)-engine light keeps coming on-code says it's something to do with the gas pressure? changed gas cap and mechanic checked other parts of that system- haven't been able to isolate the problem. Another sympton is some sort of dull knocking under the hood on the passenger side. This knocking is not consistent, making it hard to isolate the problem. Car runs good no apparent performance problems but if anybody has some suggestion on what to look at or consider please respond.Thank You