Barcode won't scan after printing: three causes, in order

The label came out. It looks clean. The scanner at the counter refuses it.

A scanner does not see a picture — it measures the width of bars and gaps against a specification. A barcode can look perfect and be unreadable, which is why this problem is so hard to eyeball. There are three causes, and they need checking in a particular order because only one of them is free to test.

1. It was printed smaller than designed

The most common cause by a distance, and the only one you can rule out without touching the printer.

If the label arrived as an A4 or Letter page and was fitted onto a 4x6 roll, everything on it printed at roughly 48% of its intended size. Bars designed at a width the scanner can resolve are now half that.

Check it in seconds:

1. Open the original PDF in Preview and press Cmd+I.
2. If the page reads 210 x 297 mm or 8.5 x 11 in, this is your cause.
3. Measure the printed label against a ruler. A 4x6 label carrying a shrunken page is unmistakable once you look for it.

The fix is not a print setting. The label has to be cut out of the page at full size rather than squeezed into it — our free browser tool does that without rescaling, so the barcode keeps exactly the dimensions the carrier drew.

2. The print density is too low

If the size is right, the next suspect is heat. A thermal printer darkens the label by applying heat, and too little produces bars that are grey rather than black, with edges that fade into the gaps.

A scanner needs a clear boundary between bar and gap. Soft edges blur that boundary, and the reader either fails or misreads.

Raise Darkness or Density in the printer preferences by two steps and print a test label. Repeat if needed.

Do not go straight to maximum. Too much heat is its own failure: the bars spread into the gaps, the gaps narrow, and a barcode that is too dark fails exactly like one that is too light. There is a window, and stepping towards it beats guessing at either end.

Faded output has other causes too — old stock, a dirty head, the roll loaded the wrong way — and they are worth ruling out before blaming the density setting.

3. Dithering is breaking up the bars

This one is rarer and more confusing, because the label looks textured rather than wrong.

Dithering simulates shades of grey by scattering dots. It is the right choice for photographs and the wrong one for barcodes: applied to a solid black bar it produces a stippled bar with a ragged edge, and the scanner sees an inconsistent width.

Where the driver exposes it, set dithering to none or ordered. Look closely at a printed bar — if it is speckled rather than solid, this is your cause and no amount of density adjustment will fix it.

Quick order of testing

1. Measure the label. Free, instant, and the most likely answer.
2. Look closely at one bar. Speckled means dithering. Grey and soft means density.
3. Raise density two steps and reprint.
4. Print the printer's own self-test if all three fail — it bypasses the Mac and tells you whether the print head is the problem.

It scanned yesterday and not today

Something changed, and it is rarely the file. Thermal stock fades with heat, light and time, so an old roll can print acceptably and degrade within days. A print head that has collected adhesive or dust leaves thin unprinted lines across the bars.

Clean the head with isopropyl alcohol and a lint-free cloth, and try a fresh roll before changing any setting. Both are cheaper than debugging a driver that was working fine.