The factory transmission oil cooler on your 1999-2005 BMWE46 is a liquid-to-liquid cooling unit utilizing the engine's cooling system to cool the transmission fluid during warm outside temperatures and warm the fluid during cold outside temperatures. Though these designs are tried and true they eventually wear out and fail leading to leaking coolant into your transmission fluid. Mixing coolant into your transmission fluid can have catastrophic results and can cost thousands of dollars in rebuilding or replacing the transmission.---- The Mishimoto Replacement Transmission Oil Cooler is a direct fit to the OEM making it easy-to- install and is equivalent to the OEM Specifications of performance. This product requires no modifications and is backed by Mishshmoto's Lifetime warranty. Replace your transmission cooler and prevent premature failure.