2007 a4 2.0 running lean (vagcom)

I removed the vacuum pump about 6-weeks ago and re-installed. Now 1000 miles later get a CEL running lean. Is it possible that not replacing the o-ring could cause an air leak, and thus the fault. i don't have the exact CEL code. I did the coolant pipe and maybe 1000 miles later I get a lean CEL, connected??

Thanks
Bruce