It means check your transmission, why not start with the oil.
AT can mean Automatic Transmission.
It means that you need to check the trans computer for codes..
Transmission problem
The AT check signal light means that there is something is wrong with the transmission. A slipping transmission can be the cause of this.
Automatic transmission.
It means there is a problem with the transmission and as such you should check with your BMW dealer.
Automatic transmission.
Overheated.
Check transmission
A flashing transmission warning light can indicate a number of transmission problems. The most common problem is low transmission fluid.
P0701 Transmission Control System Range/Performance P0701 Transmission Control System Range/Performance