Check the image itself
- Make sure the file finished downloading and opens as a complete image.
- Keep the QR pattern square. Avoid stretching it, cropping its edges or covering it with a logo or text.
- Leave the clear border around the pattern. A camera needs that quiet area to find the code.
- Use enough contrast. A dark pattern on a light background is easier to read than a low-contrast or transparent image.
Check the phone and the surroundings
- Clean the camera lens and hold the phone steady at a comfortable distance.
- Move away from glare, reflections or dim light. A glossy screen or laminated print can reflect overhead lights.
- Try the phone's camera and, where relevant, the app meant to open the QR content.
- For a printed code, check a fresh print at the real scanning distance. A small preview on a large monitor may scan differently from a poster at a counter.
Payment QR codes need an extra check
A banking app may support one payment format but not another. For a Vietnam bank-transfer code, try a banking app that supports the relevant VietQR format. If the code opens, compare the bank and recipient details shown by the app before paying. A scan does not prove that a payment has been made or received.
If a payment image fails repeatedly, recreate it from the verified bank and account details. Do not send money just to test a code.
Recreate a code when its contents changed
Static QR codes store their content. If a URL, Wi-Fi password or bank account detail has changed, make a new code and replace the old image. A printout cannot update itself.