Audi A6 4.2 quattro

VAG-COM fault code 17544, 17546, 17538

They translate to:

17538 P1130 Long Term Fuel Trim mult. Bank2 System too Lean VW
17544 P1136 Long Term Fuel Trim Add.Fuel Bank1 System too Lean VW
17546 P1138 Long Term Fuel Trim Add.Fuel Bank2 System too Lean VW

Any recommendations - Is this a simple sensor replacement issue or maybe a vacuum leak?

Any DIY articles to reference?

Thanks for any help.