3100 is boost shutoff, 30FF is underboost I believe. I was getting 30FF for a while and went crazy trying to find the issue. turns out it is my boost solenoids.
Usually 3100 is accompanied by other codes
SOLD --'07 E90 335i, PTF COBB E50, HFS4, ER CP, ETS 7", AA DP, Vishnu Exhaust, DCI, ACT, Nitto Nt555
SOLD '10 E92 335i M, 6MT COBB v3, ER IC, ER CP, VRSF DP, Fuel-it st2, Mfactory LSD