Ok, here's the situation... I bought a chipped 2001 S4 and it ran good. Later, the check engine light comes on and it starts running not so good (slugish throttle response, very low boost pressure (there is a guage), dirty exhaust. These are the sensor codes that it is showing:

P1127 -- Long Term Fuel Trim mult.
P1129 -- Long Term Fuel Trim mult.
P0421 -- ?
P1113 -- Bank1-Sensor1 Internal Resistance too High

The last one is an O2 sensor. So I have bought a new one and I'm trying to figure out how to get to Bank1, Sensor1 so that I can replace it. Hopefully it's really easy. Can anyone offer any advice?

Boost Leak, Do you here a hissing sound. Check the FAQ's Page if thats not it well talk again. Pressure tester will help. Check the TBB also.

I haven't noticed a hissing. I'll listen harder. Someone mentioned to me the possibility of a safe mode. Does the S4 have a safe mode that it could be running in when an important sensor such as O2 is not functioning properly?

Yeah its called limp mode. You can erase all the codes and get the comp. back to normal. I don't know how but my friends do. Damn I need a VAG COM bad!! Would help me out tonight LOL. If you don't hear a hissing which you would know if you had a hissing its very loud and annoying, You probably dont have a boost leak.

Good. Boost leak sounds bad. So do you know how to get to the O2 sensor? I'd rather fix the problems than just clear the codes.

o2's are on the exhaust system get underneath the car and change them out if thats what ur plan was. they gotta be expensive though.

FYI there are more than 1

Yeah, I think there are 4.