> ## Documentation Index > Fetch the complete documentation index at: https://docs.smartsend.io/llms.txt > Use this file to discover all available pages before exploring further. # Print quality issues > Fix faded, streaked, or unscannable shipping labels from your label printer If your labels print faded, streaked, or too dark to scan, work through the steps below in order. They run from the cheapest fix to the most expensive, and the first one solves most cases. Before you start, print the same label from a second roll if you have one. If both rolls print equally badly, the problem is the printer rather than the label stock. A barcode that scans on your own scanner can still fail on the carrier's equipment. Do not ship a label you are unsure about — reprint it. See [quality validation](/knowledge/general/labels#quality-validation) for what to check. ## Fixing print quality Adhesive residue and paper dust build up on the printhead and the rubber platen roller. This is the most common cause of poor print by a wide margin, and it shows as faded patches, streaks, or blank vertical lines. Clean both using the method your printer manufacturer specifies, then print a test label. On the ZD421 and ZD621, follow Zebra's cleaning procedure . Let the printhead dry fully before closing the printer. Clean every time you change the label roll, and the problem largely stops coming back. Calibration teaches the printer the length and gap of the labels it is loaded with. If the printer is misreading where each label starts and ends — skipping labels, printing across the gap, or feeding blank labels — calibrate it. Run it whenever you change label size or stock, and after switching to a roll from a different supplier even when the stated size is the same. On the ZD421 and ZD621, this is SmartCal media calibration . If the whole label is uniformly too light, or dark and smeared, the settings are wrong for your label stock rather than the hardware being faulty. Use the **lowest darkness setting that still gives good print quality**. Setting darkness too high makes the image print unclearly, stops barcodes scanning, and wears the printhead prematurely. A higher print speed generally needs more darkness to compensate, so change one at a time and print a test label after each change. See Zebra's guidance on adjusting the print quality and on the darkness setting . On a Zebra printer, use the [Print Quality Assistant](#finding-the-right-settings-on-a-zebra-printer) below instead of guessing one setting at a time. If the same vertical line is blank on every label after cleaning, and no darkness setting fixes it, individual heating elements have failed. The printhead is a consumable, not a fault — replace it. Print volume, darkness settings, and label abrasiveness all affect how long a printhead lasts, so there is no fixed replacement interval. On the ZD421 and ZD621, see Zebra's instructions for replacing the printhead . ## Finding the right settings on a Zebra printer Finding the right darkness and speed by trial and error is slow. For Zebra printers, the free **Zebra Nucleus Connector** app does it systematically. Its **Print Quality Assistant** prints a batch of test labels — you choose 4, 9, 16, or 25 — across different speed and darkness combinations, you pick the one that looks best, and the app applies those values to the printer. Nucleus Connector replaces the older Zebra Printer Setup Utility and is available for three platforms: Google Play App Store Zebra downloads The Windows version also bundles the printer drivers, so it is the one to install when setting the printer up on a computer for the first time. Before you start: * The printer must be reachable from the device running the app over USB, Bluetooth, or your network. * Load the label stock you actually ship on. The optimal settings are specific to the printer *and* the media, so calibrating on the wrong roll gives you the wrong answer. Run the assistant again whenever you change stock. * Save the resulting darkness and speed values to the printer so they survive a power cycle. The assistant applies your selection when you confirm it, and can also save the settings to a file. For the full procedure, see Zebra's documentation on working with the Print Quality Assistant , part of the Nucleus Connector user guide . This app is Zebra-specific. If you use another brand, look for the equivalent configuration utility from your printer manufacturer — most offer one. ## Still not printing correctly If the print is clean but the label itself is wrong — wrong size, cut off, or scaled — the problem is the print setup rather than print quality. See [Print setup](/fundamentals/print-setup) and print one of the [test labels](/fundamentals/print-setup#test-labels) to isolate whether the printer or the label data is at fault. What quality grade shipping labels need, and how to judge whether a printed barcode is good enough. Still stuck? We are here for you via email and phone.