U0003
High Speed CAN Communication Bus (+) Open
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Code U0003 indicates that the CAN High (+) wire on the vehicle's high-speed communication bus has an open circuit. The CAN bus uses two wires—CAN High and CAN Low—that work together as a differential pair to transmit data between modules. When the CAN-H wire is broken or disconnected, modules on either side of the break can no longer communicate with each other, potentially disabling multiple vehicle systems simultaneously.
This is a serious condition because the high-speed CAN bus connects safety-critical systems. Depending on where the open circuit occurs, you could lose engine control, transmission function, ABS braking, airbag readiness, or power steering assist. Multiple warning lights will typically illuminate, and the vehicle may enter a protective limp mode or refuse to start altogether.
Diagnosis involves tracing the CAN-H wire through the vehicle's wiring harness to find the point of the break. A multimeter set to measure resistance can help identify where continuity is lost. Common failure points include areas where the harness passes through body panels, near connectors exposed to moisture, and sections vulnerable to rodent damage. Once the break is found, repairing or replacing the affected wiring section typically resolves the issue.
Severity
Symptoms
- •Multiple warning lights illuminated including Check Engine, ABS, and airbag
- •Engine may stall, misfire, or refuse to start
- •Transmission stuck in one gear or in limp mode
- •Complete loss of instrument cluster or gauge readings
- •Power steering assist may be lost
- •Vehicle electrical accessories behaving erratically
Likely Causes
The CAN High (+) wire carries the dominant voltage signal on the bus. A physical break—from rodent damage, rubbing against a metal edge, or a pulled connector—creates an open circuit that splits the bus into isolated segments.
A connector that has vibrated loose, suffered water damage, or has a bent/broken pin on the CAN-H circuit will create an open in that specific wire while the CAN-L wire may remain intact, triggering this specific code.
Many vehicles route the CAN bus through junction blocks or star connectors. A failed solder joint or corroded splice point specifically on the CAN-H line will open the circuit at that junction.
Rarely, the CAN transceiver inside a control module can fail in a way that opens the CAN-H line, effectively disconnecting it from the bus and creating an open circuit condition.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Trace the CAN-H wire from end to end looking for physical breaks or damage
- Inspect all CAN bus connectors for bent, broken, or corroded pins on the CAN-H circuit
- Check CAN bus junction blocks and splice points for open connections
- Measure resistance on the CAN-H line to locate the open circuit point
- Repair or replace the damaged wiring section and verify bus communication is restored