Tire pressure sensor
 

I just bought my A8. I have a tire pressure warning light that came on with no display message. All tires appear to have correct inflation. I cannot read any #'s on the MMI leading me to believe that the sensor is deffective. How do I know which one is deffective? DO I need to replace all 4?

The pressures won't show up in the MMI until you drive 20 minutes after you store the pressures. The dealer can find out which sensor is malfunctioning if it has a problem.