The fault code P0451 in your Mercedes-Benz indicates a problem with the Fuel Tank Pressure Sensor, also known as sensor B4/3.

The fault code P0451 in your Mercedes-Benz indicates a problem with the Fuel Tank Pressure Sensor, also known as sensor B4/3. This code specifically points to a "Short circuit to positive" in the sensor's circuitry. Here's what you should do to diagnose and potentially resolve this issue:

1. **Inspect Wiring and Connectors:** Start by visually inspecting the wiring harness and connectors associated with the Fuel Tank Pressure Sensor (B4/3). Look for any damaged or frayed wires, loose or corroded connectors, or any signs of physical damage.

2. **Check Sensor and Its Wiring:** Examine the sensor itself and its wiring. Ensure that the sensor is securely connected and that the wiring leading to the sensor is in good condition. If the sensor or its wiring is damaged, it may need to be replaced.

3. **Clear Fault Code:** After conducting the visual inspection and addressing any identified issues, use an OBD-II scanner or a Mercedes-Benz diagnostic tool to clear the fault code (P0451). This will reset the system and turn off the Check Engine Light if the issue has been resolved.

4. **Test the Fuel Tank Pressure Sensor:** If the fault code reappears after clearing it, you should perform further testing on the Fuel Tank Pressure Sensor (B4/3) using a multimeter or a diagnostic tool capable of sensor testing. Check the sensor's resistance and voltage readings to ensure they are within the specified range.

5. **Check for Vacuum Leaks:** A vacuum leak in the fuel vapor system can affect the sensor's readings. Inspect the fuel vapor lines and components for leaks or loose connections.

6. **Inspect the Fuel Cap:** Sometimes, a loose or damaged fuel cap can trigger a P0451 code. Make sure the fuel cap is securely tightened and not damaged. Consider replacing it if it's found to be faulty.

7. **Replace the Fuel Tank Pressure Sensor:** If the sensor itself is found to be faulty during testing, you may need to replace it. Be sure to use a genuine Mercedes-Benz replacement sensor for compatibility and reliability.

8. **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.

It's important to address a P0451 fault code, as it can affect the proper functioning of the vehicle's emission control system. Additionally, always consult your vehicle's service manual or repair documentation for model-specific guidance, as the exact location and specifications of the Fuel Tank Pressure Sensor can vary between Mercedes-Benz models.