What are fault codes 04120 and 05661 on an Audi A6 2.7 TD1?

Fault code 04120 on an Audi A6 2.7 TD1 typically relates to issues with the fuel system, often indicating a problem with fuel pressure or delivery. Fault code 05661, on the other hand, is usually associated with the vehicle's electronic control unit (ECU) and may indicate a communication issue or a malfunction in a specific sensor or component. It's important to use a diagnostic tool to confirm the exact nature of these faults and to check for any additional codes that could provide further insight into the problem.