I bought an Audi A4 2.5L 132kW (2002) as second hand car. It uses way too much Diesel, over 10L on average, even when driving like a 90-year old.
Following error codes are given by its ECU:

*** ENGINE ***
01375 - Engine Mounting Valve - Stage 1 : Output does not switch/short circuit to B+
00575 - Intake Manifold Pressure : Control Limit Exceeded
01442 - Misfiring due to lack of fuel

*** CLIMATE CONTROL ***
00819 - High pressure Sensor : Short circuit to ground

As far as I can tell, the error code 00575 seems interesting in the regard of high diesel consumption.
This site: 00575 - Ross-Tech Wiki tells me 2 possible causes for this error:

• Solenoid Valve for Boost Pressure Control (N75) defective
• Turbo Charger defective

Does anyone of you know if a defective N75 valve or a defective turbo charger would have an impact on the diesel consumption?
I must say, the car drives pretty well, I think that with a defective turbo charger it would feel as if "underpowered" - therefore my suspicion is that it's the N75 valve.

 

If its your Turbo you would definitely feel loss of power, recently my wastegate was an issue, once I fixed it the performance came back. Considering your is a diesel I'm certain you would notice a heavy performance issue... - just my two cents worth