The diagnostic trouble code P0128 in your Mercedes-Benz indicates a problem with the thermostat's heating element for the two-disk thermostat.

The diagnostic trouble code P0128 in your Mercedes-Benz indicates a problem with the thermostat's heating element for the two-disk thermostat. Specifically, it reports that the component R48 (the heating element) is jammed in the open position, which results in the coolant temperature rising too slowly. The thermostat is responsible for regulating the engine's operating temperature.

Here's how to address this issue:

1. **Inspect the Thermostat Heating Element:** Begin by inspecting the thermostat's heating element (component R48). Check for any visible signs of damage, corrosion, or blockages that may be causing it to jam in the open position.

2. **Test the Thermostat Operation:** You can use a diagnostic tool or scanner to check the thermostat's operation. It should open and close smoothly when commanded by the vehicle's engine control module (ECM). If it's not operating correctly, it may need to be replaced.

3. **Check for Proper Coolant Flow:** Ensure that there are no obstructions in the coolant passages or hoses leading to and from the thermostat housing. A blockage can affect the coolant flow and cause slow temperature rise.

4. **Inspect Coolant Level and Quality:** Verify that the coolant level is at the recommended level and that the coolant itself is in good condition. Low coolant levels or degraded coolant can impact the thermostat's performance.

5. **Check the Thermostat Housing:** Inspect the thermostat housing for any damage, leaks, or signs of wear. A damaged housing can affect the thermostat's operation.

6. **Replace the Thermostat Heating Element:** If the heating element (R48) is found to be faulty or jammed beyond repair, it will need to be replaced with a genuine Mercedes-Benz replacement part.

7. **Clear Fault Code:** After addressing the issue, use a diagnostic tool to clear the fault code (P0128). This will reset the system and turn off the Check Engine Light if the problem has been resolved.

8. **Monitor Coolant Temperature:** After clearing the fault code, monitor the coolant temperature during driving to ensure that it is rising to the appropriate operating temperature within a reasonable time.

9. **Professional Assistance:** If you are unable to diagnose or resolve the issue on your own, or if the problem persists after inspection and basic maintenance, it's advisable to seek assistance from a qualified Mercedes-Benz technician or mechanic. They have the specialized knowledge and equipment needed for in-depth diagnostics and repairs related to the thermostat and cooling system.

Proper engine cooling is essential for the optimal performance and longevity of your vehicle. Addressing a malfunctioning thermostat promptly can prevent overheating and other potential engine issues. Always consult your vehicle's service manual or repair documentation for model-specific guidance.