If the whole ECU is bad, then this is true. However just one small circuit can go bad in an ECU, causing one small item on the vehicle not to work, while everything else works fine. I had this happen on my Infiniti G20. Car ran great, but check engine light came on. EGR code. Tested out everything and finally traced the problem back to the ECU. Turns out the ECU was getting all the right inputs, but when it was supposed to provide a ground to operate the EGR, it wouldn't. All grounds were good, just something internal in the ECU. Another ECU solved the problem instantly, as I watched it respond on a multi-meter.