turbo ?

I have the summit racing rich/lean gauge it has about 12bars, in normal driving the gauge brounce up and down about 4 to 5 bars when i get on it and the turbo kicks in the gauge goes lean SOMETIMES or it will stay the same never runs rich.
i have ATP chip and 3.5 fpr.
i though it should run richer then that.
is the o2 sensor bad is there a way i could tell ?
later