The tag fails inside a car
A car puts metal behind almost every surface, adds a phone mount that is often magnetic, and may have a windscreen with a metallic coating. Three separate causes share one small space, which is why car deployments fail in ways that look inconsistent.
Three causes in one cabin
Metal behind the trim. A dashboard looks like plastic and is built on a steel structure, and door cards, pillars and consoles are the same.
The phone mount. Magnetic mounts rely on a metal plate stuck to the phone or to its case, and that plate sits between the coil and everything else.
The glass. Some windscreens carry a metallic coating for heat rejection or heating, which behaves like a conductive layer.
Because all three can be present at once, changing one thing often does not fix it, and the deployment appears random.
The mount is usually the culprit
A magnetic mount needs a ferrous plate. If it is adhered to the back of the phone or inside its case, it is directly over the NFC coil.
That stops tag reading outright, in the car and everywhere else. The clue is that the phone also fails on tags at home.
Test by removing the plate, or by testing that phone away from the car with a known good tag. If it fails there too, the mount is the whole problem and the car is innocent.
Clamp and cradle mounts without magnets avoid this entirely, which is worth knowing before choosing a mount for a phone you tap with.
Coated windscreens
Windscreens with a metallic coating are common on modern vehicles, and the coating typically leaves an uncoated zone for toll transponders and similar devices.
A tag applied to coated glass has a conductive layer behind it and behaves accordingly.
If the vehicle documentation marks a zone for transponders, that zone is the part of the glass that is not coated, and it is where a tag has the best chance.
Side windows are usually less of a problem than the windscreen, and glass thickness still adds distance.
Where a tag does work
A thick plastic trim panel with air behind it rather than structure. Test before committing, since what is behind trim is not visible.
A phone cradle or a purpose made holder, where the tag sits on a plastic part standing away from the body.
An on metal tag anywhere metal is unavoidable, accepting its thickness.
And test in the actual vehicle with the actual phone and its actual mount. A car is the environment most likely to make a bench test irrelevant.
Frequently asked questions
Why does my tag work at home and not in the car?
Because the car adds metal behind the surface and often a magnetic mount plate on the phone. Test the phone at home with the plate still fitted to separate the two.
Can I stick a tag on the windscreen?
On uncoated glass it works, and many windscreens carry a metallic coating. Look for the zone the vehicle reserves for toll transponders.
Do magnets damage the tag?
No. Tags store data electrically, so a magnet has nothing to erase. The metal plate blocks reading while it is there and harms nothing.
Is a dashboard plastic or metal?
It looks like plastic and is built on a steel structure. Behaving as if it were metal is the safer assumption until you have tested.