Code No. C1801 ACC/FCM Auto Stop light monitor: Operation
WARNING: This page is about a different car, the 2014 Mitsubishi Outlander. However, it is still accessible from the selected car via links, so may be relevant.
The ASC-ECU illuminates the stop light while the ACC or the FCM is braking the vehicle automatically. The ASC-ECU monitors whether the stoplights illuminate or go out.