Download the official XTRONS Fiber Optic Decoder Box Series Overview & Connection Guide (PDF):
Click to Download
Check the following:
1. Vehicle Model & Year Compatibility
- The decoder boxes are compatible only with specific vehicles/systems.
- If unsure, please share your vehicle model and year with us for confirmation.
2. Fiber Optic Port in Your Vehicle
- Make sure the MOST connector is present and accessible.
- Please share a photo of your vehicle’s fiber optic port if you need assistance.
3. Fiber Optic Ring Integration – Common Cause of Audio Issues
- In vehicles equipped with MOST or D2B fiber optic systems, an open or incomplete ring can result in system instability, including no sound output or intermittent audio dropouts.
- Please ensure that the fiber optic circuit is fully closed and properly terminated. Even a single missing link in the loop will cause the system to malfunction.
- If original control units — such as the OEM radio or CD changer — have been removed or bypassed, a fiber optic loop adapter (shorting plug) is often required to restore signal continuity.
XTRONS solutions for MOST systems:
1. Shorting loop (model ICE/ACS/fiberloop): Keep the original fiber optic wiring and remove the original CD changer. The shorting loop bypasses the CD changer, maintaining continuity in the fiber ring.
2. Replacement fiber optic cable (model ICE/ACS/FEXL009, recommended): Replaces the original fiber optic line entirely for better stability and performance.
For D2B systems:
- XTRONS does not provide a dedicated shorting loop for D2B.
- If needed, you can purchase a D2B fiber optic loop adapter or bypass plug using these search terms:
- “Mercedes D2B Fiber Optic Loop Adapter”
- “D2B Optical Bypass Plug / Loop Connector”
- Note: The CD changer may be located either behind the dashboard (center console) or in the trunk area, depending on your vehicle model. Be sure to check both possible locations.
- For reference, please see the installation video for the XTRONS IAP92M220S head unit and D2B fiber optic decoder FOBB06 in a Mercedes S-Class:
Watch on YouTube
4. Vehicle-Specific Wake-Up Logic
• Some Mercedes-Benz models have different CAN/D2B/MOST wake-up behavior.
• In these vehicles, the decoder box may not automatically wake up when the ignition is turned on.
• Common symptoms:
- No audio on startup
- Audio works only after interacting with the head unit or another control
Suggested checks:
• Verify vehicle model and year for known wake-up differences.
• Check D2B / MOST connection and ensure the loop is fully closed.
• For vehicles with optical fiber wake-up issues, try connecting an external audio source via AUX to confirm decoder functionality.
Note: This behavior is normal for some models and does not indicate a defective decoder.
5. Function Test with External Audio Device
- Try connecting an external audio device (e.g. smartphone) via AUX to the decoder box.
- This helps confirm whether the decoder box is outputting any audio signal at all.
- Check the attachment below for a sample test connection image:
- test fobb.png (448.67 KiB) Viewed 2345104 times