The ECU reads if you have a cat or not thanks to the lambda sensors. YOu have one pre-cat and one post-cat, to analyze your exhaust fumes. If both give the same results, then the ECU understands that you don't have a cat. If the first reads pollution and the second less pollution, it understands that you have a cat. If the first reads pollution and the second reads purely clean and cold air, it considers that there is a big problem and puts everything in a safety mode. Sometimes, for some cars, it means you can't start anymore. Which kind of makes sense, because that could mean that your exhaust is broken, which is a big problem.
In case of a decat, on recent cars, we usually install a mini-cat which plugs where you had the second lambda sensor, and then you plug your lambda sensor onto that mini-cat. This way, your exhaust flow in unaltered, but at the same time your lamdba sensor read cleaner air than the first sensor.
