Laser Printer Vertical White Streaks: Diagnose the Laser Scanner Unit
A technician is troubleshooting a laser printer that produces pages with vertical white streaks. The toner cartridge has been replaced with a new OEM cartridge, and the imaging drum is clean. A test page printed from the printer's control panel also shows the streaks. Which of the following components is MOST likely causing the issue?
⚠ Common exam trap
It's easy for candidates to assume vertical white streaks are caused by a dirty or damaged drum or toner cartridge, but the question explicitly states those have been replaced, forcing you to consider the laser scanning assembly as the next logical component in the image formation process.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Laser scanning assembly
Vertical white streaks on a laser printer page indicate that the laser beam is being blocked or obstructed before it reaches the drum. Since the toner cartridge and imaging drum have been replaced and the issue persists, the most likely cause is a dirty or obstructed laser scanning assembly, which prevents the laser from exposing certain areas of the drum.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Fuser assembly
Why it's wrong here
The fuser assembly melts and presses toner into the paper using a heated roller and a pressure roller. If the fuser temperature is too low, toner does not bond, leading to smudging or smearing that can be rubbed off. If a roller is damaged, it may cause jams or a repeating defect pattern at intervals related to roller circumference, but not a narrow, continuous white line. Vertical white streaks originate before fusing, during image formation, not after toner is already applied.
When this WOULD be correct
A technician troubleshooting a laser printer that produces pages with toner smudging or ghosting, and the toner cartridge and imaging drum are known good. A test page shows the same smudging, indicating the fuser assembly is likely worn or damaged.
- ✗
Transfer roller
Why it's wrong here
The transfer roller applies a positive charge to the paper to pull toner from the photoconductor drum. If it is dirty, worn, or has uneven pressure, toner transfer becomes inconsistent, leading to light or patchy output, often in broad regions or repeating intervals, not sharp vertical white lines. A vertical white streak specifically points to an obstruction or defect in the laser light path, not the transfer process.
- ✓
Laser scanning assembly
Why this is correct
The laser scanning assembly sweeps a modulated beam across the photoconductive drum. When a contaminant such as toner dust, paper dust, or an oily residue accumulates on the rotating mirror or the lenses, it blocks or scatters the beam at a consistent point in the scan. Because the drum rotates and each scan line corresponds to a specific horizontal position, the interference yields a single narrow vertical white streak along the entire page length. Cleaning or replacing the LSU is the standard remedy.
- ✗
Paper pickup rollers
Why it's wrong here
Paper pickup rollers are mechanical parts that grip the paper stack and feed sheets into the print path. When they are worn or dirty, they slip and cause misfeeds, multiple-sheet feeds, or paper jams, but they have no role in forming the printed image. Since the image is created on the drum before paper touches it, any defect in the pickup roller cannot produce a pattern on the page itself. Thus, vertical white streaks are a symptom of an optics or drum issue, not a paper transport problem.
When this WOULD be correct
Paper pickup rollers would be the correct answer if the issue were paper jams, misfeeds, or multiple pages feeding at once. For example, a question describing repeated paper jams or skewed printing would point to worn or dirty pickup rollers.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 220-1101 exam frequently reuses these exact scenarios with slightly different constraints.
✓Laser scanning assemblyCorrect answer▾
Why this is correct
The laser scanning assembly sweeps a modulated beam across the photoconductive drum. When a contaminant such as toner dust, paper dust, or an oily residue accumulates on the rotating mirror or the lenses, it blocks or scatters the beam at a consistent point in the scan. Because the drum rotates and each scan line corresponds to a specific horizontal position, the interference yields a single narrow vertical white streak along the entire page length. Cleaning or replacing the LSU is the standard remedy.
✗Fuser assemblyWrong answer — click to see why▾
Why this is wrong here
The fuser assembly uses heat and pressure to bond toner to paper; vertical white streaks typically indicate a laser scanning or toner delivery issue, not a fuser problem. Fuser issues usually cause smudging, ghosting, or toner not fusing properly.
★ When this WOULD be the correct answer
A technician troubleshooting a laser printer that produces pages with toner smudging or ghosting, and the toner cartridge and imaging drum are known good. A test page shows the same smudging, indicating the fuser assembly is likely worn or damaged.
Why candidates choose this
Candidates may confuse fuser-related defects (like streaks or marks) with laser scanner issues, or they may assume any print defect after toner replacement is due to the fuser, especially if they have encountered fuser problems in other scenarios.
✗Paper pickup rollersWrong answer — click to see why▾
Why this is wrong here
Paper pickup rollers are responsible for feeding paper into the printer, not for image formation. Vertical white streaks are caused by a defect in the imaging process, such as a dirty or damaged laser scanning assembly, not by paper handling components.
★ When this WOULD be the correct answer
Paper pickup rollers would be the correct answer if the issue were paper jams, misfeeds, or multiple pages feeding at once. For example, a question describing repeated paper jams or skewed printing would point to worn or dirty pickup rollers.
Why candidates choose this
Candidates may confuse paper path issues with image quality problems, or they might think that any 'roller' in the printer could cause streaks, not realizing that pickup rollers only affect paper movement, not the image itself.
Analysis generated from the official 220-1101blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1101 exam.