Our C4 UrS-car's AAN has an oxygen sensor that the ECU uses to control the amount of fuel the engine is given (and the resulting emissions that are produced). Here is some info about this G39 O2 sensor:



The computer uses the O2 sensor as one of the feedback devices and adapts to the given driving conditions, as explained here:



If there are issues with the O2 probe, you might notice a decrease in fuel economy and/or power. You might get a Check Engine Light (CEL). Pulling the codes would provide this info:

2342 (Blink Fault Code), 00525 VAG1551/2 Fault Code Number
Oxygen Sensor (G39)
Sensor Wiring or Sensor defective Heat resistor in sensor defective, Signal wiring shorted to ground, Fuel tank empty, Ignition system component failure, air leak at mass air flow sensor, air leak in exhaust pipe before catalyst, poor ECU ground wire connection at intake manifold.
Symptom: Emissions not within specifications, increased fuel consumption, rich exhaust, spark plug fouling,
Carbon Monoxide (CO) percentage above 3%
(Reference SJM Autotechnik)

The G39 O2 sensor has a finite life and will need to be replaced periodically,e.g. every 60K miles, or so. The OE Part No. is 034906265F (Bosch 0 258 003 957), as shown here: