When a laser printer's toner does not fuse properly to the paper, the fuser assembly is often responsible because it is the component that applies the heat and pressure necessary to melt and bond the toner to the paper. If the fuser has become faulty, maybe due to reaching its end-of-life or sustaining damage, it won't produce the required heat to complete this bonding process, leading to smudgable toner on prints. While a low toner cartridge may indeed cause faint prints, it would not cause the toner to fail to fuse to the paper. A malfunctioning transfer belt would typically result in color alignment issues or missing colors, not fusing errors. Cold background usually indicates excess toner is present all over the paper, not just where the image forms.