basically the way it works is.. first sensor reads the fueling and adjusts it.
second checks how well the cat is working.. if its tied under the car it reads fresh air "the cat is working well then".. if its pluged into the cat then it reads the c02 at a high reading thinks the cat has failed and nocks up the light to warn you
how is it squashed? when straping the 2nd sensor under the car it must be plugged in, or it will throw the light as it cant read the emissions