My 01 TT has a both these codes. I just had a 303 and had to replace several parts before finally replacing the wiring harness. Now I have both these codes. The 138 is an 02 sensor which isn't serious but the Long Term Fuel Trim Add.Fuel,Bank1 System too Lean. I'm just wondering if I replace the O2 Sensor will it get rid of the other codes as well? If not I know it could be something as little as a MAF to a leak in the intake. I checked all lines and they all seem good so I'm just hoping it's an 02 sensor problem. Any help would be great.