17521/P1113 - Oxygen (Lambda) Sensor; B1 S1: Internal Resistance too High
17539/P1131/004401 - Oxygen/Lambda Sensor Bank 2 Sensor 1: Internal Resistance too High
17544/P1136/004406 - Fuel Trim: Bank 1 (Add): System too Lean
17546/P1138 - Fuel Trim: Bank 2 (Add): System too Lean
16839/P0455/001109 - EVAP System: Large Leak Detected
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Ok, let's start with the OXS codes first. Audi's are really good about being correct when they store an OXS code. Unless the wiring is damaged going to the sensor, replace both Bank one and Bank two OX Sensors. They are both the Pre-catalyst Sensors mounted directly behind the cylinder heads. The electrical connectors will be black in color. They are the same sensor so you will need two. Audi #078 906 265M, Bosch # XXXXX ONLY USE AUDI OR THE BOSCH SENSOR I LISTED. DO NOT BUY A CHEAP AFTERMARKET SENSOR.
The EVAP large leak code is typically a gas cap that is missing, loose or failed so check that first. If it is there and tight you might even just replace it and see if the code returns as you don't have a gas cap tester and they are relatively cheap.
As far as the lean codes go they are both Add (additive). In Audi speak this means at idle. Without seeing all the fuel trim data in MVB #32 (Measuring Value Block). I can't say for certain, but that is where a vacuum leak would show up in the Add data. With the top engine cover removed inspect all the vacuum lines, especially going down the left (driver's) side of the intake manifold and going to the front of the intake manifold. These hoses typically shrink, leak, and cause other codes. Repair any problems detected. If you don't see any problems start the engine and idle, then inspect for a "hissing" sound to try and find the source of the leak and repair if necessary.
Make the recommended repairs, clear the codes, and see what returns.
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Jake “The Audi Doctor”