In laser printing, the fuser unit is responsible for bonding the toner to the paper using heat and pressure. If the toner is not sticking to the paper, resulting in illegible documents despite trying different brands of paper, it indicates a problem with the fuser unit. It might not be reaching the necessary temperature to melt the toner properly, necessitating its replacement to ensure proper printouts. References: * CompTIA A+ Core 1 Exam Objectives Section 3.7: Given a scenario, install and replace printer consumables.