Most likely they've been caught with memos saying to install a device that would trick past the test. It doesn't really matter what the devices did on an engine

Most likely they've been caught with memos saying to install a device that would trick past the test. It doesn't really matter what the devices did on an engineering level or even whether they worked at all, and the regulator may not even know or care; the hard part in a case like this tends to be showing criminal intent.