Lost Communication with Headlamp Control Module B
Diagnostic trouble code U0242 indicates that the vehicle's control module (typically the body control module or BCM) has lost communication with the headlamp control module 'B' over the CAN (Controller Area Network) bus. This 'B' module usually corresponds to the passenger-side headlamp control module or a secondary module, depending on the vehicle architecture. The loss of communication can be caused by interruptions in the CAN bus wiring, faults in the receiving module, or power supply issues to the headlamp module.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Faulty headlamp control module 'B'
- Open or short circuit in the CAN bus communication wires
- Corroded, loose, or damaged connector at the headlamp module 'B'
- Power or ground failure to the headlamp module 'B'
- Faulty body control module (BCM) or master bus module
- Incorrect CAN bus resistance or damaged terminators
3. Symptoms You Will Notice
- Headlights may not operate correctly or at all
- Check Engine Light or other warning lights illuminated
- Possible communication errors with other modules
4. Step-by-Step Diagnosis
Scan for all stored codes and note any related CAN bus codes
Inspect the headlamp module 'B' connector for corrosion, damage, or looseness
Check the CAN bus wiring for opens, shorts, or high resistance
Verify power and ground at the headlamp module 'B'
Test the CAN bus termination resistors and overall bus resistance
If all else is good, suspect a faulty headlamp module 'B' or BCM
5. Frequently Asked Questions
Is it safe to drive?▼
The severity is medium. You may still be able to drive, but headlight function could be compromised, reducing visibility and safety. It's recommended to diagnose and repair promptly.
How much does it cost to fix?▼
Costs vary widely depending on the root cause. A simple connector repair might cost $50-$150, while replacing a headlamp control module can range from $200 to $600 or more, including labor.