The ten-dollar adapter, explained
The little dongle everyone calls "an OBD2 adapter" is really an ELM327 — originally a chip from a small Canadian company, ELM Electronics, whose entire job was translation. Cars speak several low-level electrical protocols; computers speak plain text. The ELM327 sits between them: you send it human-readable commands like ATZ (reset) or 010C (give me engine RPM), and it handles the electrical voodoo, returning hex you can actually parse. It turned car diagnostics from an engineering project into a serial-port conversation, and it's why a web page can do this at all.
why the clones are a lottery
The original chip's firmware got copied — extensively, cheaply, and with variable competence. Today's ten-dollar dongles claim "ELM327 v2.1" almost universally, and that version number is frequently a lie printed on a chip that behaves like a hurried v1.5 with bugs. The consequences are real but rarely fatal: some clones drop characters at speed, some don't support every protocol, some misreport their own capabilities. For reading fault codes and basic live data — the honest 95% of what people want — even the cheap ones usually work fine. For serious work, the genuine article or a reputable branded adapter (the twenty-to-thirty-dollar tier) earns its money.
the one spec that decides everything for browsers
Here's the buying advice that matters, and the listings do not make it easy:
1 – USB adapters — work with the bay via Web Serial, on Chrome or Edge on a computer. The most reliable option; a cable is nobody's favourite but it never fails to pair.
2 – Bluetooth LE ("BLE", "Bluetooth 4.0") — work via Web Bluetooth, including on Android phones. Look for those exact words in the listing.
3 – Classic Bluetooth — the most common and cheapest kind, and no browser can reach it. This is not a limitation of this page; browsers deliberately don't expose classic Bluetooth serial ports. Buy one of these and you'll need a phone app instead, which is precisely the thing we're trying to avoid.
4 – WiFi adapters — they create their own hotspot and speak over TCP, which browsers also can't open directly. Nice for iOS apps, useless here.
So: USB or BLE. Say it twice before adding to basket, because "Bluetooth" alone on a listing usually means the classic kind.
the polite way to treat one
Plug it in with the ignition off, then turn the key to run (engine on or off — most reads work either way). Pull it out when you're finished: cheap adapters keep a trickle of current flowing even parked, and a fortnight of that has flattened many a battery, which is a comedic way to acquire a second car problem while diagnosing the first.