You eliminate all the other possibilities. Engine Computers are normally the least suspect item but the first thing do-it-yourselfers jump on because they aren't doing what they're expected to do. Most troubleshooting procedures involve a diagnostic guide with pages of troubleshooting steps covering just one symptom or fault code that end with finding the break in a wire or replacing a defective sensor. Most of the time you need a scanner to command certain functions on and off to test the computer's ability to process incoming information and to act on it. When we read that someone replaced a bunch of parts including a computer, 95 percent of the time the computer didn't solve the problem and just added more variables to the problem. The exception is older GM vehicles. They did have a lot of computer trouble. Older mechanics who experienced those common failures assume today that's still a big problem so the first thing they want to try is the computer.