The fault code U012087 typically indicates a communication issue between various control modules in the Mercedes diesel engine. This code is specific to Mercedes-Benz vehicles.

To resolve this issue, you can start by checking the wiring connections between the control modules and the main engine harness. Look for any loose or damaged connectors, wiring, or terminals.

If the wiring seems fine, it could be a problem with one of the control modules itself. In this case, you may need to have the modules tested and potentially replaced by a qualified technician or an authorized Mercedes-Benz dealership.

Additionally, it may be helpful to clear the fault code and see if it reoccurs. Sometimes, a temporary glitch can trigger a fault code, and clearing it may resolve the issue. However, if the code keeps reappearing, it is recommended to address the underlying problem to prevent any potential engine performance issues or additional fault codes from occurring.

Overall, diagnosing and repairing a specific fault code like U012087 requires proper equipment and expert knowledge. If you are not experienced with Mercedes diesel engines, it is recommended to consult with a professional mechanic or take your vehicle to a trusted automotive service center for a thorough diagnosis and repair.