What does it mean?
OBD II fault code U0401 is a generic code that is defined as “Invalid data received – engine control module (ECM), but sometimes as “Invalid Data Received from ECM (Engine/Electronic Control Module) /PCM (Powertrain Control Module) “A””, and is set when a communication fault occurs between two or more control modules. Note that code U0401 is also sometimes further defined by a sub-code in this manner – U0401.XX.
In these cases, the sub-code serves to define the particular system in which the fault is present. However, since not all manufacturers employ sub-codes, always refer to the manual for the affected application to determine the exact definition of code U0401 as it applies to that particular application.
In general terms though, all control modules on a modern application are interconnected via a highly complex web of wiring harnesses that is collectively known as the CAN (Controller Area Network) bus system. During normal vehicle operation, operational information and commands that affect all aspects of the vehicles’ operation are continually being exchanged between control modules, some of which are programmed to expect certain types of information from closely related modules.
In almost all cases, the PCM acts not only as a sort of clearing house for exchanged information, but it also “supervises” the exchange of information between control modules and should it detect a communication failure between two or more control modules for a period of time that exceeds a limit set by the manufacturer, it will set code U0401, and illuminate a warning light.