Plug in a code reader. If you get no response, bad ECM or a bad wire/fuse going to or from it. But you may get symptoms of stalling,lack of power,bad fuel milage, and many more things that just don't seem right. But the code scanner will be you best angle of attack.f you get no response, bad ECM or a bad wire/fuse going to or from it. But you may get symptoms of stalling,lack of power,bad fuel milage, and many more things that just don't seem right.