  Outflows generated by protostars heavily affect the kinematics and chemistry
of the hosting molecular cloud through strong shocks that enhance the abundance
of some molecules. L1157 is the prototype of chemically active outflows, and a
strong shock, called B1, is taking place in its blue lobe between the
precessing jet and the hosting cloud. We present the Herschel-PACS 55--210
micron spectra of the L1157-B1 shock, showing emission lines from CO, H2O, OH,
and [OI]. The spatial resolution of the PACS spectrometer allows us to map the
warm gas traced by far-infrared (FIR) lines with unprecedented detail. The
rotational diagram of the high-Jup CO lines indicates high-excitation
conditions (Tex ~ 210 +/- 10 K). We used a radiative transfer code to model the
hot CO gas emission observed with PACS and in the CO (13-12) and (10-9) lines
measured by Herschel-HIFI. We derive 200<Tkin<800 K and n>10^5 cm-3. The CO
emission comes from a region of about 7 arcsec located at the rear of the bow
shock where the [OI] and OH emission also originate. Comparison with shock
models shows that the bright [OI] and OH emissions trace a dissociative J-type
shock, which is also supported by a previous detection of [FeII] at the same
position. The inferred mass-flux is consistent with the "reverse" shock where
the jet is impacting on the L1157-B1 bow shock. The same shock may contribute
significantly to the high-Jup CO emission.
