Why did my QR code stop working?

First, find out what the code contains

Before you change anything, read the code. What it contains tells you which of the problems below you are dealing with.

1. Scan the code with your phone camera. If nothing happens, take a clear photo and upload it to an online QR scanner that shows the content before opening it.
2. If no scanner can read it at all, the problem is the print or the design. Skip to the sections on size, contrast and damage.
3. If it reads, look at the address. Is it your own link, or a short link on a QR code company's domain?
4. Open the address in a private browser window, or on a phone that is not signed in to your accounts, to see what the public sees.

The free trial or subscription ended

This is the most common reason a QR code stops working months after it was printed. Many generators make dynamic codes by default. The code holds a short link to their server, and the server forwards each scan to your address. When a trial ends, a card payment fails or an account is closed, the forwarding stops. People scanning see an error, a page asking the owner to upgrade, or sometimes an advert.

You cannot turn a printed dynamic code into a static one, because the pattern only contains the provider's link. Your options are to log back in and renew, or to reprint with a static code that points at your own address. If you reprint, choose a code that holds your link directly so the same thing cannot happen again.

The link behind the code is broken

A static code never expires, but it is only as good as the address inside it. If the code reads fine and the page does not load, check these:

• The page was moved or renamed during a website update. Set up a redirect from the old address to the new one.
• The domain lapsed. Renew it, or the code will lead to a parked page or someone else's site.
• A Google Drive, Dropbox or OneDrive file was deleted and uploaded again, which creates a new link. Replace files using the upload-new-version option instead.
• Sharing was switched from anyone with the link to private.
• A Google Form stopped accepting responses, or was restricted to your organisation.
• A shortened link was deleted, or the shortening service closed.
• There was a typo in the address when the code was made. Scan it and compare it letter by letter.

It is printed too small or holds too much

A code that scans on your screen can fail on paper. Print it at least 2 × 2 cm, and for anything read from further away, make it roughly a tenth of the scanning distance: 10 cm wide for a poster read from a metre away.

Long addresses make the pattern denser, with smaller squares that blur together when printed small. Shorten the link, for example by using a short page on your own site, and the code gets simpler and easier to read.

Low contrast and inverted colours

Scanners look for dark squares on a light background. Pale codes, such as light grey, pastel or yellow, often fail, especially under warm indoor lighting. Gradients that fade to a light colour at one corner cause the same problem.

Inverted codes, light on a dark background, are a frequent cause of "works on my phone, not on theirs". Many recent phone cameras can read them, but a good number of scanner apps and older phones cannot. A dark code on a light background works everywhere.

Watch transparent backgrounds too. A code with a transparent background placed on a dark photo or a coloured panel has lost its light background, and with it most of its contrast.

Logos, margins and print damage

• The logo is too big, or was placed on top of the code without raising error correction. Shrink it or remake the code in a generator that adjusts error correction for you.
• The quiet zone was cropped. Codes need a clear margin of about four of their small squares on every side. Text, borders or artwork inside that margin confuse scanners.
• Glossy lamination or a shiny sticker reflects light straight into the camera. Use a matt finish where the code sits.
• The code wraps around a bottle or tube, or sits across a fold or seam, so part of it is distorted.
• A small PNG was enlarged for print and the edges went soft. Use SVG or PDF, or a much larger PNG.
• The code was stretched out of proportion when it was resized.

Wi-Fi and contact codes that go out of date

Some static codes stop working because the information inside them is no longer true. A Wi-Fi QR code contains the network name and password. Change the password and the code still offers the old one, so phones fail to connect. A contact card code keeps the phone number and email you gave it. In both cases, make a new code with the new details and replace the old one.

How to make a replacement that keeps working

1. Decide what the code should open, and use an address you control if you might want to change the destination later.
2. Make a static QR code, so the link is stored in the code itself with no redirect or trial.
3. Keep it dark on a light background with a clear margin all round, and keep any logo small.
4. Download SVG or PDF for print, or a large PNG.
5. Print a proof at the real size and scan it with two different phones from the distance people will stand.