Does a phone case block NFC?
An ordinary plastic or leather case costs a little range and rarely prevents a read. What stops NFC entirely is metal: a plate for a magnetic car mount, a metallic decorative finish, or a contactless card carried in a slot at the back of the case.
Thickness costs range
Plastic, silicone, leather and fabric are transparent to a 13.56 MHz magnetic field. They do not absorb it and they do not reflect it. What they do is hold the phone antenna further from the tag.
Since coupling collapses sharply with distance, a few extra millimetres are not nothing. A rugged case can turn a tag that read instantly into one that needs two or three attempts, and it can push a marginal tag over the edge into never working.
The tell is that the tag still works when read without the case. If removing the case fixes it, the case is costing range and the answer is either a thinner case or a bigger tag.
Metal stops it
A metal plate glued inside a case for a magnetic car mount is the single most common cause of a phone that suddenly stopped reading tags. It sits directly over the antenna, and it detunes it exactly as a metal surface detunes a tag.
Cases with metallic decorative panels, metal bumpers wide enough to cover the antenna area, and cases lined with a conductive layer behave the same way. So does a popsocket with a steel base plate.
The failure is complete rather than partial, and it affects everything: reading tags, paying, opening doors. If a phone stopped doing all three at once, look at the case before anything else.
The card slot problem
Cases with a pocket for cards create a subtler failure. A contactless bank card sitting in that slot is a coil directly over the phone antenna.
Two things follow. The phone struggles to read tags, because the card is competing for the field. And when the phone is presented to a terminal, the terminal may see the card, the phone, or an unresolvable mess of both, which is why some payment terminals reject a phone in such a case.
This one is worth knowing because the owner rarely connects the two. The card was added for convenience months before the payment problems started.
How to test yours
Four steps, in order, and stop as soon as something works.
Read a tag with the case on. Then remove any card from the slot and try again. Then remove the case entirely and try again. Then try a different spot on the back of the phone, since the case may be shifting where you naturally place it.
If it only works with the case off, the case is the problem and no setting will fix it. If it works with the card removed, the card was the problem. If nothing works even bare, the phone or the tag is at fault and the case was never involved.
Frequently asked questions
Will a thicker case always cause problems?
Not always, and it makes marginal cases fail. A large tag read through a thick case can be perfectly reliable, while a small sticker through the same case may not work at all.
Can I remove the metal plate from my case?
Often yes, since they are usually glued rather than moulded in. If the plate is needed for a car mount, moving it away from the antenna area is sometimes enough, and testing after each attempt is the only way to know.
Does a magnetic mount damage the phone?
Magnets are not the issue for NFC, the metal plate is, because it is a conductor over the antenna rather than because it is magnetic. Magnetic card mounts affect magnetic stripes on cards, which is a separate concern.
Why does my phone still pay but not read tags?
Card emulation and reading are not equally sensitive, and the terminal usually has a larger and stronger antenna than a tag does. A partial obstruction can leave payment working while tag reading stops.