P0137 says O2 sensor is reading to low a voltage (lean) and remaining below 40 Mv . Since voltage is changing could be the sensor. To eliminate wiring swap left to right O2's and see if it moves (P0157) is B2S2. What is the good side B2S2 voltage reading now.
I'm Bob.. 1994 Z28, A4, Stall, CAI, Cam, RR's, 342 rear, LT's , CatBack, Tuned