Can an OBD tell you if your catalytic converter isn’t working properly?
Only if your car is OBD-II, which has a second oxygen sensor after the convertor. Generally cars 1996 and newer are OBD-II.
If OBD-II…Yes it can…HOWEVER…many times it’s a false reading. If the O2 sensor is bad it’ll send a signal saying the cat is bad…when it’s actually the O2 sensor.