U0004 - CONTROLLER AREA NETWORK (CAN) DATA BUS: HIGH-SPEED BUS (+) - VOLTAGE LOW

The car controllers are connected to the CAN Bus (Controller Area Network) and through this network, the transmission of work data and other parameters that the modules share with each other is processed. Each module has stored messages that are required at the time of communication, which are constantly monitored. When a receiving controller receives such a message, it checks that the transmitting controller is available.

The communication networks are HEV High-Speed Controller Area Network (HS-CAN), High-Speed Controller Area Network (HS-CAN), Medium Speed Controller Area Network (MS-CAN), and Infotainment Controller Area Network (I-CAN). If the High-Speed Controller Area Network (HS-CAN) is malfunctioning at any given time, the DTC U0004 OBDII  will be set.

• Check Engine Lamp illuminated.
• Inconveniences in the car’s performance, ranging from failures in the speed increase to a malfunction of the ABS. It all depends on the module that has difficulties in sending data.
• The car alarm activates suddenly.
• The car presents difficulties when starting.
• The engine suddenly shuts off.

The reasons for the U0004 OBD2 diagnostic code being stored are:

• Other DTCs may be stored.
• Controller wiring or connections may be faulty.
• A control module may be in poor condition.
• There is probably a short circuit in the CAN Bus High-Speed Controller Area Network.
• The power supply on the CAN Bus could be deficient.
• The software on a control module may be out of date.

In order to correct the U0004 OBDII fault code consider the following advice:

• Consult the TSBs.
• Verify with a scan tool if other DTCs are stored in addition to the U0004 code. Pay attention to codes that indicate problems with a specific controller or component. Correct it as applicable.
• Reset the error and observe if it returns. If it reappears, inspect for any oddities in the wiring or junctions. On locating any deterioration in the harness, it should be remedied and then the system rescanned to exclude the fault code again.
• Check the condition of the connections of the control modules connected to the High-Speed Controller Area Network (HS-CAN). If you discover common damage, proceed with the appropriate adjustments.
• Analyze the damage by unplugging the modules one by one when monitoring the network with the scanner. If a module is unplugged and restarts the network communication, it is a symptom that there is a cut or poor signal with that module.
• When confirming that a certain module is malfunctioning, before replacing it, it should be tested to ensure that it has a flawless power supply and an optimal ground connection.
• It is advisable to restore the software of the faulty module. In certain cases, the modules can be reprogrammed.