Code Definition

U0078 — Control Module Communication Low Speed CAN Bus Off

System: Network Communication  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About U0078

U0078 is a generic (obd-ii standard) network communication diagnostic trouble code. It belongs to the U-prefix family of codes, which covers inter-module network communication (CAN, LIN, FlexRay, or similar) faults.

When U0078 is stored, the module has detected a signal or supply voltage lower than the expected operating range.

Scanning & clearing: Network (U) codes report communication faults between modules. When diagnosing U-codes, checking the vehicle's CAN bus integrity, supply/ground at each module, and reading all modules for related faults is usually the first step.

Because U0078 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The control module monitors network communication on the low speed CAN bus to ensure valid data exchange between body related modules such as the BCM, instrument cluster, and HVAC module. On this GM based architecture the low speed bus is a single wire fault tolerant CAN network. DTC U0078 is set when the module detects that the low speed CAN bus has gone bus off, meaning communication has been lost for a calibrated period because the bus voltage levels are incorrect, the line is open or shorted, or a module on the network has failed and is disrupting bus operation. When this code is set the d...

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