If your car's ECU is bad, there are a few things that you will notice. First, the check engine light will likely be illuminated on your dash. Second, your car may run rough or stall completely. Third, you may notice reduced fuel efficiency. Finally, your car may have trouble starting. If you notice any of these symptoms, it's important to take your car to a mechanic to have it diagnosed. They can then determine if the ECU is the root of the problem and replace it if necessary.