This return line hose carries exhaust fluid from the injector or dosing module back to the tank in the selective catalytic reduction system. It completes the closed loop that recirculates unused reagent, preventing crystallization and maintaining system pressure. A cracked or blocked hose causes the system to log a fault and derate engine power. The hose is formed to the same routing path as the original assembly, which keeps it clear of hot surfaces and moving components. On replacement, ensure the quick-connect fittings are fully seated and the retaining clips are locked.