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DTC codes / U01xx · Lost communication with a control module / U018A

U018ALost Communication With Headlamp Leveling Control Module B

U018A is a generic network communication code indicating that one or more control modules have lost the CAN (Controller Area Network) messages from the headlamp leveling control module B. The computer detected that the expected periodic data frames from this module stopped arriving on the bus, which can be caused by a power, ground, or bus wiring fault rather than a failed module.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaLost communication with a control module

Symptoms you may notice

  • The headlamp leveling warning light or a general network fault indicator is illuminated on the dashboard.
  • The headlamps may point too low or too high, or the automatic leveling feature stops working.
  • Other unrelated warning lights or module communication codes may appear if the module shares a bus segment.
  • Often no symptoms besides the warning light, especially if the headlamp aim is not noticeably affected.
  • In some vehicles, the headlamp leveling system defaults to a fixed position and the manual adjuster has no effect.
  • Intermittent warning light that comes and goes with vibration, moisture, or temperature changes.

Common causes, most likely first

  1. A blown fuse or poor connection in the power circuit feeding the headlamp leveling control module B.
  2. Corroded, loose, or damaged ground connection for the headlamp leveling control module B.
  3. Open or shorted CAN bus wiring between the module and the network, often due to chafing or rodent damage.
  4. A faulty headlamp leveling control module B itself, though this is less common than wiring issues.
  5. Water intrusion into the module connector or housing, causing intermittent communication loss.
  6. A weak or failing battery or charging system that drops voltage below the module's operating threshold.
  7. A network gateway or central communication module that is not routing messages correctly, though this usually sets additional codes.

How to diagnose and fix it

  1. Verify battery voltage and charging system output, then clear the code and test-drive to see if it returns immediately.
  2. Check all fuses and relays that supply power to the headlamp leveling control module B, and inspect the connectors for corrosion or looseness.
  3. Measure resistance from the module ground pin to the chassis and repair any high-resistance or open ground path.
  4. Inspect the CAN bus wiring at the module connector for opens, shorts, or chafing, and perform a resistance check across the bus with the battery disconnected.
  5. If wiring and power are good, substitute a known-good module of the same part number to confirm whether the original module is faulty.
  6. After any repair, clear the code and perform a network relearn or initialization procedure if the manufacturer requires it for the headlamp leveling system.
  7. Road-test the vehicle and confirm that the headlamp leveling system operates correctly and the code does not reset.

Why headlamp leveling module B goes offline

  • A common misdiagnosis is replacing the headlamp leveling module when the real fault is a corroded ground stud behind a fender liner or under the hood.
  • This code can be set by a low-voltage condition during engine cranking, so always check for a weak battery before condemning the module.
  • Some vehicles use a shared bus segment for headlamp leveling and other body modules, so a single chafed wire can set U018A along with several other U-codes.
  • The module may communicate perfectly on a bench test yet fail intermittently in the vehicle due to vibration or water intrusion at the connector.
  • U018A does not always mean the module is dead; it only means the network stopped hearing from it, and a failed power relay can produce the same code.

Can I keep driving with U018A?

You can usually drive the vehicle with U018A stored, but the headlamps may not aim correctly at night or in load conditions, so avoid night driving until the fault is repaired. If the warning light is accompanied by other communication codes or drivability issues, have the vehicle inspected promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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