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Case Study: Mercedes Keyless Go Not Working on W205 C300
If your Mercedes displays a Keyless Go not working issue doors not unlocking, no response to the Start/Stop button, or complete ignition failure you’re not alone.
This real workshop case study from a Mercedes-Benz W205 C300 (2015) demonstrates how a simple CAN Bus communication fault can shut down the entire Keyless Go system.
To troubleshoot similar electrical failures and CAN Bus issues, explore our main hub: Mercedes Electrical Problems: Fix SAM, ECU & CAN Bus Faults
Vehicle & Customer Complaint
Model: Mercedes-Benz W205 C300
Symptoms reported:
- – The Keyless Go unlock function did not work.
- – The Start/Stop button did not respond.
- – Central locking system unresponsive at times.
- – No crank, no noise, no ignition recognition.
The owner noted the issue started intermittently but eventually became permanent. The vehicle could no longer start without using the physical key blade.
Step 1 : Full System Scan Using XENTRY
A diagnostic session was launched using STAR XENTRY Diagnosis.
Multiple fault codes appeared, all pointing toward:
- – Communication errors between Keyless Go Control Unit (N69/5)
- – CAN-B CAN-C gateway communication loss
- – Unreachable Keyless Go antennas
Conclusion:
This was not simply a key fob battery issue.
It was a network-level communication fault.
Step 2 : Check Power Supply to the Keyless Go Module
The technician verified:
- – Terminal 30 (permanent power): OK
- – Terminal 15 (ignition wake-up): OK
- – Ground points: clean and secure
Why this step matters:
A Keyless Go module with poor power supply acts “dead” and blocks door unlocking and engine start.
But in this case, voltages were correct. So the investigation moved deeper.
Step 3 : CAN BUS Integrity Testing
Because Keyless Go relies on the CAN network to communicate with:
- – Front SAM
- – Rear SAM
- – ESL (electronic steering lock)
- – ISM (ignition switch module)
- – ECU
the technician checked CAN High (CAN-H) and CAN Low (CAN-L):
Findings:
- – CAN-H value unstable
- – CAN-L reading dropped intermittently
- – CAN waveform asymmetrical on oscilloscope
- – No continuity in one of the CAN loops
This confirmed a network communication interruption.
Step 4 : Tracing the Wiring to the CAN Distribution Point
Mercedes models often have CAN distributors hidden under:
- – Footwells
- – SAM modules
- – Under seats
- – Behind trim
For the W205, the CAN BUS distributor related to Keyless Go is positioned near the front passenger side wiring channel.
Critical Discovery:
A broken CAN BUS connector at the distributor point
This connector is essential for allowing the Keyless Go module to communicate with the rest of the vehicle’s electrical network.
A broken connector =
→ No communication
→ No module wake-up
→ No unlocking
→ No start authorization
→ Keyless Go completely disabled
This is a classic electrical failure often caused by:
- – Poor previous repairs
- – Moisture
- – Connector fatigue
- – Incorrect reassembly after interior work
Step 5 : Repairing the Fault
The technician:
- 1. Removed the damaged CAN distributor connector
- 2. Cleaned the wiring terminals
- 3. Installed a new OEM CAN connector
- 4. Secured the harness to avoid future strain
After reconnecting:
- – CAN High and CAN Low returned to normal values
- – Network communication restored instantly
- – No more fault codes for Keyless Go module
Step 6 : Post-Repair Validation
A full verification process was performed:
✔ Central locking now responds to the key
✔ Start/Stop button activates immediately
✔ Engine cranks and starts normally
✔ No more CAN-related fault codes
✔ Keyless Go antennas operational
✔ SAM modules communicating correctly
The Keyless Go system was restored to 100% functionality.
Case Study Summary
This case illustrates a key lesson in Mercedes electrical diagnostics:
Most Keyless Go failures are NOT caused by the key or the module itself.
They are caused by CAN BUS communication faults.
Key Takeaways:
- – Always check voltage and ground first
- – CAN BUS testing is mandatory for Keyless Go diagnosis
- – A single broken connector can shut down the entire system
- – Avoid replacing expensive modules before verifying communication
- – XENTRY fault codes showing “communication error” nearly always indicate wiring or connector faults
FAQs
1. What is Mercedes-Benz Keyless Go?
A hands-free access and ignition system allowing you to open, lock, and start the car while the key stays in your pocket.
2. How does Keyless Go work?
Uses RFID technology, antennas in door handles/interior, module communication via CAN BUS, and a coded handshake with the key fob.
3. Why does Keyless Go stop working?
Common causes:
- – CAN BUS failure
- – Faulty door handle antenna
- – Damaged wiring in doors or footwell
- – Weak 12V battery
- – Electronic steering lock faults
- – Damaged Keyless Go module
4. Can a broken connector really disable Keyless Go?
Yes, because the module becomes invisible on the CAN network.
5. Do I need XENTRY to fix Keyless Go?
For full diagnostics, yes.
For wiring repairs, not necessarily.
Conclusion
The Mercedes keyless go not working issue in the W205 C300 was ultimately caused by a broken CAN BUS connector, not the module or key fob.
Once the connector was replaced and communication restored, the Keyless Go system worked flawlessly.
This case demonstrates the importance of:
- – Proper electrical diagnostics
- – Understanding CAN BUS communication
- – Avoiding unnecessary module replacements
For owners and technicians, this is a reminder that small electrical faults can completely disable major systems in modern Mercedes cars.
Author Bio
Written by Mercedes Expert
With years of hands-on experience diagnosing and repairing Mercedes-Benz systems, he brings technical depth and practical case studies to help car owners, technicians, and enthusiasts troubleshoot complex automotive issues. His work focuses on clear repair guides, OEM-level procedures, and knowledge-sharing to empower both professionals and drivers.
Last update: November 2025
— Salim, Mercedes Expert
Independent specialist in Mercedes-Benz diagnostics, CAN Bus analysis, troubleshooting case studies, and EV systems.