The fault code P179A77 in your Mercedes-Benz indicates a malfunction in the redundancy system for the DIRECT SELECT gear shifter.

The fault code P179A77 in your Mercedes-Benz indicates a malfunction in the redundancy system for the DIRECT SELECT gear shifter. This code suggests that there is an issue with the gear selector mechanism or the system that controls it, making it unable to reach the commanded position.

Here are some steps to help diagnose and potentially address this issue:

1. **Inspect the Gear Selector Mechanism:** Start by visually inspecting the gear selector mechanism, including the shifter itself and the components that link it to the transmission. Look for any obvious signs of damage, such as broken or loose parts, disconnected linkages, or debris interfering with the shifter's movement.

2. **Check for Obstructions:** Ensure that there are no obstructions or foreign objects blocking the movement of the gear selector. Sometimes, objects can get lodged in the shifter area, preventing it from reaching the intended position.

3. **Check for Proper Gear Engagement:** Verify that the vehicle is properly engaging the selected gear when you shift. Make sure the transmission is responding correctly to the gear selector commands.

4. **Check Transmission Fluid Levels:** Low or contaminated transmission fluid can affect the operation of the transmission and gear shifting. Check the transmission fluid level and quality, and top it up or replace it as needed.

5. **Clear Fault Code:** After addressing any identified issues, use a Mercedes-Benz-specific diagnostic tool or an OBD-II scanner to clear the fault code (P179A77). This will reset the system and turn off any warning lights if the issue has been resolved.

6. **Test Redundancy System:** If the fault code reappears after clearing it, you may need to perform further diagnostic tests on the redundancy system for DIRECT SELECT. This might involve using specialized diagnostic equipment and procedures to determine the specific nature of the malfunction.

7. **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 DIRECT SELECT system.

It's important to address this issue promptly, as a malfunction in the gear selector system can affect the drivability and safety of your vehicle. Additionally, always consult your vehicle's service manual or repair documentation for model-specific guidance, as the exact components and procedures involved can vary between Mercedes-Benz models.