QR code troubleshooting

Something isn't scanning. Find the cause, in order of likelihood.

• Calendar QR code saves the wrong timeFloating DTSTART times land differently in every timezone. The three ways ICS writes a time, broken and fixed payloads, and when all-day events are safer.
• Email QR code body loses its line breaksmailto bodies need %0D%0A line breaks per RFC 6068, %20 spaces and %2B plus signs. The encoding traps, the density cost, and verified payload examples.
• I lost the original QR code fileThe payload is recoverable from any print in one scan. The styling is not. The full rebuild recipe, why the new code may look different, and prevention.
• Payment QR code amount not prefilledUPI carries the amount in am=, PIX in EMV tag 54, EPC in an EUR line. Why the field goes missing, why some apps ignore it, and how to regenerate per scheme.
• QR code blocked by corporate or guest WiFiCaptive portals, category filters and newly-registered-domain rules block QR destinations on office and venue networks. Diagnosis and fixes for both sides.
• QR code blocked by Safe Browsing or SmartScreenThe red warning page means the destination domain is flagged, not the code. Why shared shorteners inherit other users' abuse, and how to check and fix it.
• QR code blurry when printedBlurry printed QR codes come from raster upscaling, not the printer. The resample chain explained, and how SVG or exact-size regeneration fixes it.
• QR code broken by image compressionJPEG ringing, chat-app recompression and low-quality WebP all eat module edges. Why compressed QR codes fail, and the PNG-or-SVG rule that prevents it.
• QR code changed after I edited the imageEditing pixels never edits the payload, but it can destroy the code: rotation, resizing and recolouring all corrupt modules. Edit the source state instead.
• QR code colours not scanningDecoders convert the image to greyscale before deciding dark from light, so what matters is luminance difference, not how different the colours look. Two…
• QR code cropped by a printerPrinters leave a 3–5 mm unprintable margin, and borderless modes enlarge the page. Why edge-placed QR codes get clipped, and safe-area rules to fix it.
• QR code cut off in a PDFArtboard clipping, masks and transparency flattening can all break QR codes in PDFs. The 400% zoom check, and export settings that keep vectors intact.
• QR code disappears in black-and-white printingColour pairs with strong on-screen contrast can convert to nearly the same grey. Why QR codes vanish in mono print, and the greyscale test that catches it.
• QR code goes to a dead domainA lapsed domain can be re-registered by anyone, including an attacker who inherits your printed codes. Rescue timelines, monitoring and prevention.
• QR code inverted and unreadableQR codes are defined dark-on-light. Inverted codes scan on some apps and fail on others, for no benefit. How to spot polarity problems and fix them fast.
• QR code link shortener expiredWhen a free shortener retires or paywalls old links, every printed QR code through it dies at once. Recovery options ranked, and how to avoid a repeat.
• QR code mirrored by a design toolFlip transforms mirror QR codes silently, and transfer-print workflows invite it. How to spot a mirrored code by its finder corners, and how to fix it.
• QR code not scanning: the checklistWork through these in order, because they are ranked by how often they are the actual cause: quiet zone, contrast, size for distance, glare, blur from…
• QR code not working: a 60-second diagnosisEvery failed QR code lands in one of three classes, never detected, decodes the wrong thing, or a broken destination. A 60-second flow finds yours.
• QR code opens a 404 pageThe code decodes fine but the page is gone: moved paths, case-sensitive servers, rejected parameters. Plus the redirect rescue if you own the domain.
• QR code pixelated when enlargedUpscaling cannot add detail: bicubic blurs module edges and nearest-neighbour distorts widths. Why enlarged QR codes fail and why regenerating fixes it.
• QR code quiet zone too smallWithout a four-module quiet zone the scanner never finds the code at all. How borders, dark backgrounds and tight padding break detection, with fixes.
• QR code scans on Android but not iPhoneUsually a decoder difference, not a broken code: Android-only URI schemes, Micro QR, very dense codes or an old iPhone. Causes ranked, with fixes.
• QR code scans on one phone but not othersOne phone scanning means the code sits at the edge of decodability. Camera sampling, focus distance and a spent error-correction budget, with fixes.
• QR code scans on screen but not in printThree causes account for nearly all of these: the raster image was scaled up in the layout, ink spread on the substrate closed the gaps between modules,…
• QR code stuck in a redirect loopShortener-to-consent-to-app-link chains that bounce forever, how to trace every hop with curl -IL, and why flattening the chain fixes it permanently.
• QR code too dense to scanA dense code means too many modules for the printed size. The fix is almost always to shorten the payload rather than to enlarge the code or raise error…
• QR code too small to scan: the fix ladderCameras need 3–4 pixels per module. Minimum sizes by scan distance, why shortening the payload beats enlarging the code, and the 2 × 2 cm print floor.
• QR code with a logo not scanningA logo works up to roughly 20–25% of the code area at error-correction level H, and only in the centre. Covering a finder pattern, the timing patterns, or…
• QR codes in WebP and AVIF: what breaksLossless WebP and AVIF are safe; lossy defaults are not. The converter-quality trap, pipelines that silently recompress your PNG, and a verdict table.
• SMS QR code body not prefilled on iPhoneThe sms URI split its body syntax between platforms for years. Why SMSTO is the form that works everywhere today, with a payload test matrix.
• UPI QR code not detected by the payment appRanked causes (a malformed upi:// link with no pa, app parsing quirks, BharatQR confusion, and screenshot degradation), with the payload shape to check against.
• UPI QR code shows the wrong nameThe pn parameter is only a hint: UPI apps display the bank-registered name for the VPA. When a differing name is normal, and when it is a warning sign.
• vCard QR code saves a truncated contactQR decoding is all-or-nothing, so a cut-off contact was cut at generation. The density maths that forces the cut, and the URL pattern that avoids it.
• WiFi QR code has the wrong security typeThe T field accepts exactly WPA, WEP or nopass: there is no WPA2 or WPA3 token. What each value means, why mismatches fail silently, and how to fix them.
• WiFi QR code not connectingThe usual causes, in order: an unescaped special character in the password or SSID, a hidden network without the hidden flag, choosing WEP or nopass when…
• WiFi QR code special characters: the escaping rulesBackslash, semicolon, comma, colon and double quote must be escaped in WiFi QR SSIDs and passwords. Broken and fixed payloads, plus the hex-quoting rule.
• QR code opens the wrong link or a 404Decode the code first to see the string it actually contains. If the string is right, the problem is the destination: a deleted page, a retired shortener,…
• vCard QR code imports with missing or wrong fieldsTwo causes cover almost all of these: unescaped commas and semicolons inside a value, which split the field, and a missing FN line, which leaves the…