The exhaust system on a 2003–2004 5.9L Cummins is designed to route, cool, and clean the gases produced during combustion. It begins at the exhaust manifold, which collects gases from the engine’s six cylinders and channels them into the turbocharger to help boost intake pressure. From there, exhaust flows through downpipes into the mid-pipe and muffler, eventually exiting through the tailpipe. The system may also include sensors like EGT (exhaust gas temperature) probes and, in some setups, a catalytic converter, although emissions equipment was minimal in this era. Made from high-temp steel or stainless materials, the exhaust components must withstand extreme heat and pressure. Many owners upgrade the system with larger-diameter piping, performance mufflers, or turbo-back kits to reduce backpressure, lower EGTs, and improve flow for increased power and efficiency.