SCP (J1850) Invalid or Missing Data for Primary Id

OBD II fault code U1101 is a manufacturer specific code that is defined as “SCP (J1850) Invalid or Missing Data for Primary Id”, and is set when the PCM (Powertrain Control Module) detects a break or interruption in communication between itself and the primary control module in a given system, which in the case of code U1101, is the transmission control module on most applications.

NOTE: Note that General Motors defines code U1101 as “Loss of Serial Communications for Class 2 Devices”, which amounts to much the same thing as the definition given above. SPECIAL NOTES: It should be noted that there are several dozen manufacturer specific trouble codes that have the “SCP (J1850) Invalid or Missing Data for Primary Id” definition.

In all cases, these codes can be decoded in the following manner- “SCP” refers to the Standard Corporate Protocol, which is one of several communications protocols in use by different vehicle manufacturers “J1850” refers to the SAE standard that describes the SCP communications protocol “Primary Id” refers to the identification code each control module in a CAN (Controller Area Network) serial bus communications system has.

When a vehicle is in operation, all the control modules in a vehicle communicate with each other via the CAN bus system. During data transmissions, each control module sends its unique identification code along with its data transmission, and other control modules use this identification code to identify the source of the data transmission.

Therefore, and in the context of code U1101, the Primary Id refers to the transmission control module as the originator of a data transmission, hence the reference to “Primary Id”. It should be noted though that while many codes referring to Primary Id’s identify the relevant control module, most do not, which makes it imperative that all codes in this class be researched thoroughly before a diagnosis is attempted. END OF SPECIAL NOTES.

As a practical matter, CAN bus systems are composed of hundreds of circuits that interconnect all the control modules on a vehicle. Moreover, CAN bus systems are set up in such a manner that data from a particular control module is shared with many other modules, with the PCM acting as a sort of clearing house for all data transmissions.

In the case of code U1101, which refers to the transmission control module on most vehicles, data concerning shift points, torque converter lock-up clutch control, and others is shared with several other control modules. These typically include the throttle control module (on drive-by-wire systems), the fuel control module, ignition control, and others that may include the traction-, stability-, and cruise control modules.

We need not delve into the technical complexities of the relationships between these control modules here, but suffice to say that the result of these relationships is that the transmission control module exercises a great deal of control over several engine control functions to make gear shifts more comfortable, to reduce emissions, and to protect transmission components against the effects of aggressive accelerations.

Thus, when a defect, malfunction, or failure exists between the transmission control module and the PCM that prevents data being transmitted from the transmission control module to the PCM, either or both the PCM and transmission control module will recognize that several critical engine control functions cannot be executed, and one or both control modules will set code U1101, and illuminate one or more warning lights.

Note that in some cases, the transmission may also be forced into a fail-safe or limp mode as a safety precaution.