This code indicates that the vehicle's high-speed CAN communication bus has detected a low signal condition on the CAN low circuit. It typically means one or more modules cannot communicate properly due to a wiring fault, connector issue, or a module pulling the bus low.
A chafed or damaged wire grounding the CAN low circuit will pull the bus signal low and set this code. This is the most common cause.
An internally shorted control module can drag the CAN low line down. Identifying which module requires isolation testing.
A broken wire, corroded splice, or loose terminal interrupts the low circuit between modules.
CAN junction connectors and splices are common failure points, especially in moisture-exposed areas.
A module missing its power or ground may fail to release the bus or communicate, disrupting the network.
Only considered after all wiring, connectors, and other modules are verified. The PCM itself is rarely the cause of a CAN bus low fault.
No. This is almost always a wiring, connector, or splice fault, or a different module shorting the bus. Replacing the PCM is rarely the correct fix and should only be considered after all other causes are eliminated.
It depends on which modules are affected. If critical systems like ABS or engine management lose communication, the vehicle may be unsafe to drive. Have it diagnosed promptly.
Because the CAN bus is shared, a single low-circuit fault can prevent many modules from communicating, generating several U-codes from one root cause.
No. U0004 is a network communication code, not an emissions code, though loss of communication to certain modules may trigger secondary emissions-related codes.