U0043: Vehicle Communication Bus B Low Fault
The U0043 code indicates a communication failure on the Vehicle Communication Bus B Low line. This code is a generic network error per SAE J2012, meaning an electronic control unit (ECU) has detected an interruption, out-of-range voltage, or excessive noise on the low signal wire of the secondary communication bus designated as 'B'. Modern OBD-II systems use multiple CAN buses to separate critical engine communications from comfort and safety systems. A fault on the Bus B Low line prevents modules on that network from exchanging data properly, causing the OBD-II scanner to log this loss-of-communication or signal error code.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Short to ground on the Bus B Low communication line
- Short to voltage on the Bus B Low communication line
- Open circuit or broken wire in the Bus B network
- Corroded connector or bent pins on any module in the B network
- Internal failure of a control module (ECU) connected to Bus B
- Faulty or out-of-spec CAN termination resistor (if applicable to the secondary bus)
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Intermittent or complete loss of function from modules on Bus B (e.g., infotainment, climate control, or safety systems)
4. Step-by-Step Diagnosis
Scan all modules for related communication codes and note which modules are offline.
Visually inspect the Bus B wiring and connectors for damage, corrosion, or bent pins.
Measure resistance across the CAN bus termination resistors (typically 60 ohms on a properly terminated bus).
Check for shorts to ground or power on the Bus B Low wire using a multimeter.
If wiring is intact, isolate modules one by one to identify a faulty ECU causing the bus fault.
5. Frequently Asked Questions
Is it safe to drive?▼
The severity is medium. You can usually drive short distances, but some comfort or safety features may not work. Have it diagnosed soon to avoid further issues.
How much does it cost to fix?▼
Costs vary widely: from $100-$200 for a simple wiring repair to $500-$1000+ if a module needs replacement. Diagnostic fees are typically $100-$150.