Hi, this is the overview of your code error.
possible an air leak if the manifold has been removed or a leaking hose, i'd be checking all the hoses etc for tightness to start with then working from there for leaks .
if all ok then go to next items.
16555/P0171/000369 - Fuel Trim; Bank 1: System Too Lean
Possible Causes
Mass Air Flow Sensor (G70) defective
Intake System Leak(s)
Possible Solutions
Check Mass Air Flow Sensor (G70)
Check Fuel Pressure Regulation
Check Fuel Pump
Check Injectors
Check Intake System for Leaks
Check Exhaust System for Leaks
Check Secondary Air System for Leaks
Check Vacuum Pipes for Leaks
also this .
P0171 Audi Description
With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios.
In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the Malfunction Indicator Light (MIL) (2 trip detection logic).
IT may seem mad but has the MAF sensor plug been refitted properly, also worth a check just to eliminate connection issues.