Is your adapter lying: a five-minute check
A cheap clone answers briskly but cannot do half the protocols. Here is how to spot it.
See how the adapter introduces itself
The app shows the version the adapter itself reports. «ELM327 v2.1» on a cheap Chinese board is almost always untrue: genuine 2.x chips are rare and priced differently. On its own this is not a verdict, but it is the first reason to distrust what it says.
Check the battery voltage it reports
The adapter must report voltage, and it must match reality: about 12.5 V with the engine off and 13.5-14.5 V with it running. A flat «12.0 V» in every condition means the number is invented, not measured.
See how many protocols it can actually do
A genuine ELM327 handles all nine OBD-II protocols. Clones often manage only the common 11-bit CAN and stay silent on the rest. On a newer car you will not notice; on an older one such an adapter simply «finds no» units.
Tell a bad adapter from a silent car
The tell-tale: the adapter answers its own service commands briskly and instantly, but every question to the car returns «no data». A healthy adapter on a healthy car answers more slowly, because it waits for the unit to reply.
Do not repair a car on data from a doubtful adapter
A clone can silently round and drop values. A «replace the sensor» decision made on such a reading costs more than a new adapter. If the numbers look odd, first check what you measured them with.