This is from the 2001 Prius Diagnostic Section for the HV Battery ECU:
"No serial signals transmitted from HV ECU"
"Trouble Area-Communications bus line/HV ECU"
"Check for open and short in wire harness between HV ECU and Battery ECU"
Bus connection at the HV ECU is
H13 pin2 (White HTD+)
H13 pin20 (Black HTD-)
Bus connector at the Battery ECU is
B17 pin 6 (White HTD+)
B17 pin 7 (Black HTD-)
There is an intermediate junction connector IF2 where HTD+ is pin 1 and HTD- is pin 2. IF2 is on the fire wall where the little panel at the hinge of the drivers side door pulls out. It is not one of the accessible junction block connectors, but is the 6 pin connector on the fire wall way deep in. Chances are you have a loose or corroded connector/pin. Ensure that they are all clean as well as checking for opens or shorts.
the HTD probably stands for HV To Battery ECU. There is a pair of signals called DTH for the reverse signal path, but this apparently must be working. The manual also states to stop all operation of "optional electric products" to ensure EMI interference is not causing the problem.