What Does U0242 Mean?
The Engine Control Module (ECM) or Body Control Module (BCM) has lost communication with the secondary headlamp control module on the vehicle's network bus. This typically indicates a network wiring fault, module power issue, or failed headlamp control module.
Common Causes
35%
Damaged or corroded wiring harness between modules or at connectors
25%
Faulty Headlamp Control Module B
20%
Blown fuse or loss of power/ground to headlamp control module
15%
CAN bus network fault or short circuit
5%
Failed BCM or gateway module causing network dropout
Diagnostic Steps
1
Step 1: Verify all related fuses for headlamp control module B and check for proper power and ground at module connector
2
Step 2: Inspect wiring harness and connectors for damage, corrosion, or loose connections between headlamp module B and the network
3
Step 3: Using scan tool, check for network communication and verify module is present on bus using module scan function
4
Step 4: Measure CAN bus voltage between CAN-High and CAN-Low at headlamp module connector (should be approximately 2.5V at rest, 0-5V when active)
5
Step 5: If wiring and power are good, substitute or reflash headlamp control module B to verify module failure
Estimated Repair Cost
$150 - $800
Parts + labor, varies by vehicle and location
The Engine Control Module (ECM) or Body Control Module (BCM) has lost communication with the secondary headlamp control module on the vehicle's network bus. This typically indicates a network wiring fault, module power issue, or failed headlamp control module.
The most common cause of U0242 (Lost Communication With Headlamp Control Module "B") is: Damaged or corroded wiring harness between modules or at connectors
Typical repair costs for U0242 range from $150 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
May result in loss of adaptive lighting, automatic high beam, or cornering light functions. Primary headlight function typically remains operational.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0242 to identify the root cause.
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Quick Info
Category
Network
System
Lighting System / Communication Network
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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