QR code not working: a 60-second diagnosis

A QR code fails in one of three ways. The scanner never detects it (quiet zone, contrast, size), it decodes the wrong string (bad input, tampering, a mangled image), or the destination is broken (dead URL, retired shortener, blocked network). Decode the code with a scanner that shows the raw text: that one step splits the three classes apart.

The three failure classes

"Not working" hides three different faults with three different owners:

Class	What you see	Where the fault is
1. Never detected	The camera never reacts; no banner appears	The image: quiet zone, contrast, size, blur
2. Decodes the wrong thing	A link appears, but it is wrong or garbage	The data: wrong input, tampering, a mangled file
3. Destination broken	The link opens, then errors	The URL: dead page, retired shortener, blocked network

The instinct is to re-scan harder. Diagnosis is faster.

The 60-second flow

Decode the code with a tool that shows the raw text instead of following it. Our scanner does this in the browser, from the camera or an uploaded photo.
Nothing decodes at all? Class 1: the image is the problem.
It decodes, but the string is wrong? Class 2: the data is the problem.
The string is right, but the link fails? Class 3: the destination is the problem.

That single decode separates the image from the data from the destination, the split most people never make. Each class routes to its fix below.

Class 1: the camera never finds it

Ranked by how often each is the real cause:

Quiet zone. The blank margin must be at least 4 modules on every side. Designers crowd it constantly, and without it the code is never detected at all: see quiet zone too small.
Contrast and polarity. Dark modules on a light background, with at least a 40% luminance difference. Light-on-dark and low-contrast brand colours scan on some phones and not others.
Size for the distance. Printed width must be at least scan distance ÷ 10. A 3 cm code across a lobby has no chance: see too small to scan.
Blur and damage. Upscaled raster images, heavy compression, glare, folds and worn print.

The not-scanning checklist works through all of these in priority order, with the numbers.

Class 2: it decodes to the wrong thing

Wrong input. A typo, a missing https://, trailing whitespace or an invisible character in the pasted URL. Regenerate from a copied link, never a typed one.
Tampering. A sticker pasted over the original is the standard quishing move on parking meters, payment standees and menus. Feel the surface for a raised edge.
A design tool mangled it. Codes that were traced, redrawn, mirrored or flattened can decode to garbage. Regenerate rather than repair.

Full walkthrough: QR code opens the wrong link.

Class 3: the code is fine, the destination is broken

The string decodes exactly as intended; the fault is at the other end:

404 or moved page: the site was rebuilt and the old path lost its redirect.
A retired link shortener: free redirect services sunset, prune links or move them behind paywalls; see shortener expired.
A lapsed domain: the worst case, because someone else can register it next; see dead domain.
A blocking network, corporate DNS filters and captive portals commonly block shortener domains wholesale.

Test the URL in a browser on mobile data, not the office WiFi, before blaming the code.

Prevent the next one

Before anything goes to print, run the artwork through our validator. It renders the code, decodes the output and checks contrast and quiet zone, which catches classes 1 and 2 in a single pass. For class 3, point printed codes at a stable path on a domain you own, and re-test the printed estate quarterly.

FAQ

Why did my QR code suddenly stop working?

A static code's pattern never changes, so a code that used to work almost always has a destination problem: a moved page, a retired shortener or a lapsed domain. Physical wear and a sticker pasted over the original are the remaining suspects.

How do I find out what is wrong with my QR code?

Decode it with a scanner that shows the raw text rather than opening it. No decode means an image problem, a wrong string means a data problem, and a correct string with a failing link means a destination problem.

Do QR codes expire?

The pattern itself never expires. It is printed geometry. What expires is what it points to: shortened links, hosted redirects and domains all have lifespans. A direct URL on a domain you keep renewed works indefinitely.

Can a QR code be repaired without reprinting?

Only destination faults. If the code points at a path on a domain you control, fixing the page or adding a redirect heals every printed copy instantly. Image and data faults require regenerating and reprinting.