U0042: Vehicle Bus B Circuit Open
The U0042 code indicates that the engine control module (ECM) has detected an open circuit in the vehicle's communication Bus B. This means the data signal (typically CAN High or CAN Low) cannot complete its electrical path between modules on that bus, disrupting network communication.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Cut, broken, or disconnected wiring in the Bus B (CAN High/CAN Low) circuit
- Damaged, loose, or retracted pins in the control module or bus connector
- Faulty or missing Bus B termination resistor (open circuit in the 120-ohm resistor)
- Internal failure of the CAN transceiver in a module on Bus B, leaving the bus in an open state
- Short to battery or ground on one of the bus lines causing latch-up protection that simulates an open circuit
3. Symptoms You Will Notice
- Check Engine Light or other warning lights illuminated
- Communication loss with modules on Bus B, such as ABS, transmission, or instrument cluster
4. Step-by-Step Diagnosis
Use a scan tool to confirm U0042 and check for related codes on other modules
Visually inspect Bus B wiring and connectors for damage, corrosion, or loose connections
Measure resistance between CAN High and CAN Low at the bus with key off; should be approximately 60 ohms (two 120-ohm resistors in parallel). If open, check termination resistors
Check for shorts to power or ground on each CAN line using a multimeter
If wiring and resistors are good, isolate modules one by one to find the faulty transceiver
5. Frequently Asked Questions
Is it safe to drive?▼
No, driving with U0042 is not recommended. The open circuit can cause loss of critical communications between modules, potentially leading to unpredictable vehicle behavior or safety system failures.
How much does it cost to fix?▼
Repair costs vary widely: simple wiring repairs may cost $100-$300, while replacing a faulty module can range from $500 to $1,500 or more, including diagnostics and labor.