DTC U0121-00: Communication Error To DSC HU/Cm (TCM) (Us): Outline

WARNING: This page is about a different car, the 2020 Mazda CX-30. However, it is still accessible from the selected car via links, so may be relevant.

DTC U0121:00 (TCM, ET6A-EL, ET6AX-EL, US) DETECTION CONDITION AND POSSIBLE CAUSE

System malfunction location	Communication error to DSC HU/CM
Detection condition	Under the following condition, the TCM cannot receive the signal from DSC HU/CM: Battery voltage is 10 V or more.
Fail-safe	Inhibits learning control. Inhibits neutral idle control.
Possible cause	CAN communication line malfunction between TCM and DSC HU/CM TCM malfunction
System wiring diagram	Not applicable
Connector diagram	Not applicable