Hello Mercedes Benz forum members,
I’d like to ask for permission to post and seek insights from the seniors here.
Vehicle Data
Mercedes-Benz W203
Automatic transmission (by-wire)
Chronology & Symptoms
Initially, the car could sometimes be started, sometimes not
Key inserts normally
Dashboard lights turn on brightly
When turned to START → no starter click sound
A new duplicate key has been made, but:
Symptoms remain the same
There were occasions when the key could not be turned at all
Steering Lock (ESL):
A “click” sound is heard when the key is inserted
Steering wheel unlocks
Functionally, it appears to be working
Immobilizer & Scan Results
Immobilizer declared OK
Terminal 50 detected as active / working
When turned to START, a response is read on the scan tool
Scan results show CAN BUS errors across multiple modules:
CAN message missing / incorrect on several modules simultaneously
Transmission Issues
TCM / EGS is not detected by the scanner
Workshop states there is no communication between the engine ECU and the TCM
Even a dedicated transmission diagnostic computer cannot read the EGS
Impact:
Gear selector is locked in P
Vehicle cannot move forward or backward
Current Condition
Battery normal
Dashboard normal
Immobilizer OK
Terminal 50 active
Engine ECU is readable
TCM / EGS not readable at all
CAN BUS errors across modules
Transmission locked in fail-safe (P)
Questions to Fellow Forum Members
With Terminal 50 active and the immobilizer OK, but the TCM/EGS not detected, on the W203 is this more commonly caused by the K40 relay module or power & ground issues, rather than a failed TCM itself?
Has anyone experienced:
ECU readable
TCM missing from the network
Widespread CAN errors
Gear lever locked in P
and resolved it only by repairing the K40 or fixing ground connections?
On the W203, which ground points most often cause:
TCM not being detected
CAN BUS errors
Transmission stuck in fail-safe mode?
Under what conditions should the gear selector be conclusively diagnosed as faulty?
(e.g., EGS already readable normally, power & CAN stable, etc.)
Purpose of This Inquiry
I want to ensure the correct diagnostic sequence so as not to unnecessarily replace expensive parts
(TCM / selector), and to confirm whether the order K40 → power & ground → CAN → m

This image shows the detailed fault code screen of a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan during an auto scan.

This image shows a Vehicle Diagnostic Report from a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan (up to model year 08/2003). The report focuses on the ME-SFI 2.8 – Motor Electronics (Engine Control Unit) and lists 5 diagnostic trouble codes (DTCs).

This image shows the system overview screen of a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan, displaying the results of an auto scan across multiple control modules.

This photo shows the screen of a diagnostic scan tool connected to a Mercedes-Benz W203 (C240 Sedan). The scan tool is performing an auto scan and displaying a System List of vehicle control modules.l

This image shows the upper section of the System List from a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan, captured during an auto scan procedure.

This image shows another section of the auto-scan System List from a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan.

This image shows the lower section of the System List from a diagnostic scan tool connected to a Mercedes-Benz W203 C240 Sedan, during an auto scan process.
odules
is indeed the most appropriate approach for a case like this.
Thank you in advance for your insights and shared experiences 🙏

 

If the key FOB only intermittently "clicks" and sometimes cannot turn, it's probably a mechanical problem with the EIS. Take a look at this thread.

 

Check the cables going to the cam sensors on the front of the cams. There should be
little pig tail plugs. Are one or both of the cams leaking into the wire loom?
If those pig tails aren't there oil cam seep in and cause all kinds of havoc.
Same with the tranmission. There's a plug going into it (5 speed auto) if
it's full of oil pull up the carpet on the floorboard on side of the car opposite the driver,
and check the TCU. If it is full of oil, then there is your problem.The wire loom is full of oil and the TCU shorted.