Fault diagnosis
Lines and streaks down the page.
White lines, black lines and repeating marks are three different faults with three different causes. The pattern — and the distance between repeats — identifies the part.
White vertical lines usually mean toner is low or something is blocking the laser path. Black vertical lines usually mean a worn wiper blade or a scratched drum. Marks that repeat at a regular interval down the page come from a roller — and measuring the gap between repeats tells you which roller, because each one has a different circumference.
The repeat-interval trick
This is the single most useful diagnostic on a laser printer, and almost nobody outside the trade knows it.
If a mark repeats down the page at a consistent spacing, it is being made by something rotating. Every rotating part has a different circumference, so the distance between repeats identifies the part. Measure from the top of one mark to the top of the next.
Roughly: the shortest repeats come from the small charge and developer rollers, mid-range repeats from the fuser rollers, and the longest from the drum. Exact figures vary by model, which is why we measure against the specific machine rather than a generic chart — but the principle holds on every laser printer ever made.
Worth doing before you call. Print a page that shows the fault, measure the gap between two repeats in millimetres, and tell us the number. It often takes the diagnosis straight to the part.
White or pale vertical lines
- Toner low in that band — rock the cartridge and refit
- Something on the laser scanner window — dust or a fibre casts a shadow the full length of the page
- Debris on the developer roller — stops toner being picked up in that strip
- A crease or contamination in the cartridge blocking toner feed
The giveaway: a white line from a blocked laser path runs the entire length of the page with sharp edges. A white line from low toner is softer and often varies.
Black vertical lines
- Worn wiper blade — the blade that cleans leftover toner off the drum. When it fails, toner escapes past it and prints as a black line every rotation. The most common cause by a distance.
- Scratched drum — physical damage holds toner in the scratch
- Overfull waste chamber — spent toner backing up and escaping
- Contaminated fuser roller — usually smears rather than clean lines
All of these are cartridge-side rather than printer-side, which is why a black line often disappears the moment you swap in a different cartridge. That is a useful test in itself.
Horizontal bands and repeating marks
Bands across the page — rather than down it — point at a roller or the drum, and this is where the repeat measurement earns its keep. Spots, blobs or ghost images repeating at a fixed interval are nearly always a roller surface fault.
Smearing and toner that rubs off
If the print looks right but smudges when you touch it, the image is not being fused. That is the fuser — either failing to reach temperature, or with a worn or contaminated roller. It can also be paper far heavier than the machine's setting expects.
A fuser is not a user-serviceable part. If pages come out warm but smudge, stop and call.
Inkjet: banding
On inkjets the equivalent is banding — regular horizontal stripes of missing or lighter colour.
- Clogged nozzles — run a nozzle check first
- Head misalignment — run the printer's alignment routine
- Paper feeding unevenly — worn feed rollers stepping the paper inconsistently
A quick test that saves time
Print the machine's own self-test or configuration page, from the printer's panel rather than from the computer.
- Fault appears on the self-test too — it is the printer or the cartridge
- Self-test is clean — the problem is upstream: the driver, the file, or the application
That one page removes half the possibilities in about thirty seconds.
Common questions
What causes a black line down every printed page?
Most often a worn wiper blade inside the toner cartridge, which lets waste toner escape onto the drum once per rotation. A scratched drum or an overfull waste chamber will do the same. Swapping in a different cartridge is a quick way to confirm it.
How do I know which roller is causing a repeating mark?
Measure the distance between two repeats. Every rotating part has a different circumference, so the spacing identifies the part. Tell us the measurement in millimetres and it usually takes the diagnosis straight to the component.
Why does my print smudge when I touch it?
The toner is not being fused to the paper. That is the fuser — not reaching temperature, or worn. It is not a user-serviceable part, so stop printing and call.
The lines disappear when I use a different cartridge.
Then the fault is in the cartridge, not the printer — almost certainly the wiper blade or the drum. A proper refill includes checking both, which is exactly why drill-and-fill refills cause this.
Should I clean inside the printer myself?
Light dust on accessible surfaces, with a dry lint-free cloth, is fine. Do not touch the drum surface, do not use solvents, and leave the laser scanner window alone unless you know what you are looking at — it scratches easily and a scratch is permanent.
Tell us what it is doing.
Call and describe the make and the symptom — most jobs are done in a single visit, and we will tell you straight if it is not worth repairing.