U0242 — Lost communication with headlight control module B

The vehicle's main computer cannot communicate with the secondary headlight control module. This usually happens due to wiring issues, a faulty module, or network communication errors. The headlights may not function correctly or at all, and the car's system detects this loss of signal and stores the code.

Common causes

The cause is almost always one of a few things. Here they are — U0242 rarely comes from anything outside this list:

• Damaged or corroded wiring/connectors to headlight control module
• Faulty headlight control module B
• Network communication fault in vehicle's CAN bus
• Blown fuse related to headlight control circuit

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Check and clean wiring connectors to headlight module
2. Inspect and replace blown fuses if found
3. Test communication lines for continuity and shorts
4. Replace headlight control module if faulty

That is what holds for U0242 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.