CAN Bus B Low Voltage Fault
The U0040 code indicates that the control module has detected a low voltage condition on the Vehicle Communication Bus B, also known as CAN Bus B (Controller Area Network). This serial data network allows multiple electronic modules—such as the ECM, BCM, ABS, and others—to communicate. A low voltage on this bus prevents proper data transmission, leading to communication failures between modules, intermittent functions, or disabling of systems that rely on the CAN B network. Per SAE J2012, this code signifies a voltage level below the calibrated threshold on the secondary communication bus.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Damaged or incorrect resistance in the CAN B bus terminating resistor
- CAN B bus wiring shorted to ground
- CAN B bus wiring shorted to another lower-voltage circuit
- Internal control module fault affecting bus voltage
- Corroded, oxidized, or high-resistance connector or splice in the CAN B bus
- Power supply failure in a module connected to the CAN B bus
3. Symptoms You Will Notice
- Intermittent or complete loss of communication between modules
- Malfunctioning or disabled systems that rely on the CAN B network
4. Step-by-Step Diagnosis
Check for technical service bulletins (TSBs) related to U0040 for the specific vehicle.
Inspect the CAN B bus wiring and connectors for damage, corrosion, or shorts.
Measure the resistance across the CAN B bus terminals (should be approximately 60 ohms with both terminating resistors intact).
Test the voltage on the CAN B bus lines (CAN High and CAN Low) with the ignition on; typical values are around 2.5V for each line.
Isolate and test individual modules to identify which one is pulling the bus voltage low.
5. Frequently Asked Questions
Is it safe to drive?▼
The severity is medium. You may be able to drive, but expect intermittent or non-functional systems. It's best to diagnose and repair promptly to avoid further issues.
How much does it cost to fix?▼
Repair costs vary widely, typically ranging from $100 to $500, depending on whether the issue is a simple wiring repair or a module replacement.