Code U0003 indicates that the High Speed Controller Area Network (CAN) communication bus has an open circuit, meaning there is a physical break in the wiring preventing the vehicle's computers from exchanging data. This open circuit disrupts the digital network, causing modules to lose contact with each other and setting this network-level fault code.
Chafed, broken, or severed CAN bus wires (CAN High or CAN Low) creating an open circuit anywhere in the vehicle's wiring loom.
Pins pushed back, water intrusion, or loose connections at a module or intermediate connector breaking the continuous bus circuit.
One of the two 120-ohm terminating resistors is disconnected or has failed, which physically breaks the bus loop if the resistor is inline rather than parallel.
A module on the CAN bus has an internal open circuit, physically breaking the communication line inside the module itself.
A physical failure inside the Powertrain Control Module that breaks the internal CAN bus circuitry, which is exceptionally rare.
No, it rarely means the PCM is defective. Replacing the PCM for a U0003 code is a common and costly misdiagnosis. The code indicates an open circuit in the CAN bus network, which is almost always caused by damaged wiring, a corroded connector, or a disconnected terminal—not a failed computer.
A 'Bus Open' condition means there is a physical break or disconnect in the communication circuit. Just like a cut electrical wire stops the flow of power, an open CAN bus wire stops digital messages from traveling between the vehicle's computers.
It is generally unsafe and not recommended. Because the High Speed CAN bus controls critical systems like the engine, transmission, and ABS, a loss of communication can lead to stalling, a no-start condition, or unexpected warning lights, compromising vehicle safety.
The primary test involves measuring resistance between pins 6 and 14 at the OBD-II diagnostic port with the ignition off. A normal reading is roughly 60 ohms. A reading of 120 ohms indicates one side of the bus or one terminating resistor is open, while an infinite (OL) reading confirms a complete break in the bus circuit.