A 2018 Mercedes GLC Coupe rolls in with a B220600 fault code and a cluster warning that the active brake assist function is limited. The technician noted the verbatim cluster message: "Aktivt broms assistent funktion begränsad".
Our diagnostic engine analyzed this case and ranked a software fault in the CPA control unit (A90) as the most likely cause, coming in at an 85 percent probability. The diagnostic path reveals how tightly these network systems depend on timing.
The timing window that trips a B220600 code
The root issue often comes down to a known software threshold for the collision warning module. When the ignition turns on, the electronic ignition lock (EZS or N73) broadcasts the vehicle identification number over the CAN bus. The A90 module has a very narrow time window to receive and verify this message.
If the CAN message arrives even a few milliseconds late during the startup test, the module assumes the VIN is missing or invalid. This throws the B220600 code and locks the system into a fail-safe state. The fix here is an official software update to widen that timing window, followed by a reset of the learned values (autoliv_config) via the diagnostic system.
SCN coding and memory corruption
If the software level is correct, the next highest probability in our data at 75 percent is corrupted SCN coding. The A90 module stores the specific 17-character VIN and its coding string in non-volatile memory via a factory Software Calibration Number.
If the unit experiences a voltage dip, or if someone previously reset it without performing an on-line SCN coding, the internal configuration table will not match the vehicle. The module will then reject all safety-related radar functions. The diagnostic step is straightforward. Read the live data for the VIN parameter in the A90 module. If it is missing or incorrect, the unit requires a full on-line SCN coding and commissioning against the Mercedes-Benz VeDoc server.
Tracing the voltage drop
What causes that data corruption in the first place? Often, it is a supply voltage issue, which our platform ranked at a 50 percent probability for this code. The collision warning system needs a stable supply above 10.5 volts during engine cranking to maintain encrypted CAN communication with the central gateway and the ignition lock.
If the main 12V starter battery has high internal resistance, or if the auxiliary battery in the passenger footwell is degraded, the system voltage plummets during the start sequence. This voltage drop clips the CAN messages at the exact moment the A90 module requests the VIN. You can verify this by measuring the starter battery voltage drop under load with an oscilloscope or a digital multimeter set to min/max mode during a cold crank.
Ruling out hardware failure
Finally, there is a 35 percent chance of a permanent hardware or EEPROM defect in the A90 radar module. This unit integrates the radar antenna and the control logic in one housing. Over time, temperature cycles, vibration, and moisture can cause the internal EEPROM or microcontroller to fail.
When this happens, the component can no longer store the VIN. It will fail the SCN coding process, or it will revert to a fault state as soon as the control unit loses ignition voltage. The test for this is to perform an adaptation, let the vehicle sit in a sleep state for 5 minutes, and verify if the module retains its memory values.
Are you seeing this tight VIN verification window trip up other Mercedes chassis from this generation? Let me know how these cases look in your bays.