This is a code of the Diagnostic Module DM. It is for O2 sensor full rich trim (O2S1 - G3/2). It infers that the sensor is continuously reading a lean mixture even after full correction. Possible causes include air leaks, bad sensor, shorted wire etc.
Actual values should be read and the HFM controller should also be tested from faults.
Bosch Master, ASE Master, L1
33 years MB technician