The Active Exhaust System (AES)

was introduced in 2007 on Big Twin models for the non-US markets, including the UK and Australia. It is an electric motor, regulated by the engine management control unit, that opens or closes a flap in the exhaust system to comply with noise regulations.

Accessory exhaust systems do not have this motor. However, the then exposed plug produces an error code in the ECU and causes the engine warning light to come on. Both can be avoided with the AES Bypass.