My 2000 Miata check engine light says that the catalytic converter is inefficient. Maybe the sensor is wrong. How does it measure efficiency?
By the two O2 sensors.
There are 2 oxygen sensors. One in front of and one in back of the cat. The cars computer compares the two to determine efficiency. If the code you are getting is a P0420, check for leaks in the exhaust beween the engine and the cat first. Check to make sure the rear O2 sensor is working properly and only then replace the cat.
Just for your info… even if you don’t have a state emission inspection, it is a federal crime to remove the cat.