bought used audi a4 quatro 1.8t 2001 after replacing water pump and timing belt few weeks after got a code of po171 system too lean bank 1. I changed all the braded vacum hoses by the intake manifold, clean the maf sensor ,change air filter and reset the computer. but after running 30 miles the light came back. I really don't know what to do next anybody could help me please? Is the o2 sensor got something to do with this?