Bianca98: I believe the AUC sensor is an air quality sensor that monitors the air quality being fed into the car's cabin.

X-friends, please correct me if I am wrong.

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Current:
2002 X5 3.0i E53 - Max

Previous:
2003 XB9s - Rebeka(missed)
1993 525i E34 - Elizabeth(sold at 83K miles)