Takagi 110U Indoor (T-KJr2U-IN) Installation Manual And Owner's Manual page 44

Error code on the remote
controller 9008172005 (TM-RE40)
Example: If your unit has the "321" error code (which signifies an inlet thermistor failure)
Remote controller 9008172005 (TM-RE40):
Water heater:
Example: If Unit #2 has the "321" error code (inlet thermistor failure)
Remote controller 9008172005 (TM-RE40):
Parent unit:
Unit #2:
Unit #3 and #4:
ERROR CODES
-General-
110U (T-KJr2U) and 310U (T-K4U) models
Error code on the computer board
Owner's Guide
510U (T-D2U) models