A 2016 Audi A6 rolled in with a persistent buzzing noise from the left headlight and a single B149C31 code stored in the system. The technician working on the vehicle noted that the noise stopped immediately when the ignition was turned off.
Our platform analyzed this combination of the B149C31 fault code (no signal or electrical fault) and the specific physical symptom. The engine ranked an internal component failure in the left LED headlight power output stage as the most likely cause, at 85 percent probability.
The Power Output Stage Failure
The power output stage regulates the voltage and current to the LED modules using internal DC/DC converters and smoothing coils. When an internal component like a MOSFET shorts out or a coil fails, it often creates a strong acoustic resonance. You hear this as a distinct buzzing sound the exact moment the ignition turns on and terminal 15 activates. The driver fails to initialize communication or stabilize the output voltage, which sets the B149C31 code.
To prove this, the fastest diagnostic step is a cross-swap of the power output stage between the left and right headlights to see if the fault follows the module. A quick stethoscope check directly against the metal housing of the module with the ignition on will also confirm the source of the resonance.
Water Intrusion and Corrosion
The platform ranked moisture intrusion and contact corrosion at the power output stage as the second most likely cause at 70 percent. The Audi A6 (4G) chassis is known to suffer from condensation entering through the headlight sealing joints or ventilation membranes. Because the power output stage sits at the bottom of the headlight assembly, gravity does the rest. Water collects directly in the module connector and around the circuit board gasket.
This leads to galvanic corrosion and stray currents between the voltage supply pins and the LIN communication line. The result is a short circuit, the buzzing noise, and a lost signal. The diagnostic step here is straightforward. Inspect the inside of the headlight glass for water pooling, then remove the power output stage to check the contact pins and the rubber gasket for green corrosion.
Mechanical Sources of Headlight Buzzing
We also have to consider moving parts inside the housing. The diagnostic engine ranked a defective or mechanically blocked internal cooling fan at 55 percent probability. These headlights use an internal brushless fan to cool the powerful LED circuits, and it activates in standby mode as soon as the ignition turns on. If the fan bearings seize or the blades foul on internal wiring, it generates a loud buzz. Because the power output stage monitors this fan, a blockage causes an incorrect current draw or a missing RPM signal, forcing the output stage to shut down and set the code. You can verify this by inspecting the fan blades for free rotation through the rear service hatch.
Finally, the system ranked a jammed headlight range control stepper motor at 40 percent probability. During ignition-on, the vehicle runs an automatic self-test of the headlight range control. If the mechanism binds or the end-position detection fails, the stepper motor can get stuck continuously driving against a mechanical stop. This creates an endless buzzing or creaking sound. Testing this involves checking the automatic reference run against a wall and manually checking the adjustment screw for smooth operation.
Fixing the Job Right
Cross-swapping parts is standard practice, but knowing exactly which module is humming and why prevents you from replacing an expensive LED assembly when only the bottom driver is corroded. Are you seeing similar failure rates on the 4G chassis headlights in your bays?