Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Control Module
The most important task of the headlight control module (HCM) is handling the active headlight function.
The control module is mounted beneath the driver's seat. The entire control module is removed from the vehicle upon replacement.
The headlight control module (HCM) communicates both with directly connected components, and with other control modules via CAN communication.
The control module checks activations and input and output signals using an integrated diagnostic system A diagnostic trouble code is generated if the control module detects a fault. In certain cases the control module replaces the faulty signal with a substitute value.
Any diagnostic trouble codes are stored in the relevant control module memory. The data can be read off using a diagnostic tool.
If the control module detects a fault, a diagnostic trouble code is registered in the control module's internal memory. At the same time, a number of values that were frozen when the fault occurred are store. Depending on the severity of the fault, some functions will be fully or partially disabled. A warning or information text will appear in the driver information module (DIM) display. In addition, the LED in the active headlight button will flash. Diagnostic trouble codes and frozen values (detailed diagnostic trouble code information) can be read with a diagnostic tool via the data link connector in the vehicle.
In order to check whether the headlight control module (HCM) is supplied power and is grounded, the ignition switch can be set to position II.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.