Barcode not scanning: 10 causes and their fixes
Before reprinting a whole batch, check the causes below in order: the first ones are the most common and the easiest to fix.
1. The check digit is wrong
For EAN-13, EAN-8, UPC-A and ITF-14, the last digit is calculated from the others. A code copied with a single typo has an inconsistent check digit, and many scanners silently refuse to read it. The generator checks the digit and shows the expected value. The calculation is explained in EAN-13 barcodes.
2. The barcode is too small
Below 80 % of nominal size (a 0.264 mm module for an EAN-13), tills struggle to read it. Dimensions are in barcode size.
3. The bars are too short
A very flat barcode can be read by a handheld scanner held straight, but not by a checkout scanner that sweeps at several angles. Keep close to nominal height for retail products.
4. The quiet zone is blocked
Text, a cutting line or the label edge too close to the bars stops the scanner finding the start and end. Leave at least 10 modules of white each side, 11 on the left for an EAN-13.
5. The colours don’t work
Laser scanners use red light, so red, orange or light yellow bars look white to them. Safe combinations are black, dark blue or dark green bars on white or a very light background. Avoid metallic backgrounds and high-gloss varnish, which reflect light back into the scanner. On clear labels, the background is whatever is behind the label: see clear barcode labels.
6. The print was scaled
“Fit to page” shrinks the print by a few percent. That’s enough to misalign barcodes on a sheet and sometimes to push them below minimum size. Always print at 100 % (“actual size”), margins “None”.
7. The thermal printhead is dirty or worn
White lines running through every label at the same place point to a dirty or dead element on the printhead. Clean it with a cleaning pen or a swab with isopropyl alcohol, printer off and head cool. If the line stays, the head is worn. Until it is replaced, rotate the barcode 90° so the bars run across the feed direction: the white line then runs along the reading direction instead of splitting a bar, and the scanner can read the rest of the height.
8. Darkness is too high or too low
On a thermal printer, too much heat makes bars bleed and close the spaces; too little gives grey bars. Lower the print speed before raising the darkness, then test.
9. The symbology is disabled in the scanner
Many scanners ship with some symbologies off, notably ITF and certain Code 39 variants. If a well-printed barcode only scans with your phone, switch the symbology on using the scanner’s configuration guide.
10. The scanner types the wrong characters
A USB scanner behaves like a keyboard. If its keyboard language doesn’t match the computer’s, digits and letters can come out wrong, for example symbols instead of numbers on a French or German computer. Set the scanner to the same keyboard layout as the computer by scanning the matching code in its manual.
A quick test before a full run
Print one sheet or three labels, scan them with the equipment you will actually use (till, handheld scanner, stock terminal), and check that the value read matches the code exactly. Two minutes of testing saves reprinting a whole roll.