Automatic transmissions for the 2003–2004 5.9L Cummins, primarily the 48RE, are 4-speed units designed for heavy-duty use and towing. Paired with the high-torque output of the Cummins, the 48RE features electronic controls and a lock-up torque converter for better efficiency and shift quality.

While reliable in stock applications, the 48RE can become a weak point in modified or high-power setups. Upgrades like billet input shafts, stronger clutches, and tuned valve bodies are common to improve durability and performance.

Regular maintenance and quality components are key to keeping the automatic transmission responsive and capable under load.