The first concern is this code refers to something that could adversely affect emissions. Those are the codes that turn on the Check Engine light. It could have a relatively minor cause, but if it's ignored, it could turn into an expensive repair, especially if the catalytic converter overheats. Also, a totally different problem could develop that could also turn expensive if ignored. You will never know that second problem occurred because the Check Engine light is already on.
The second concern is to detect a problem, there is always a long list of conditions that must be met before a fault code will be set. One of those conditions is certain other codes can't already be set. The Engine Computer compares many sensor readings and operating conditions to each other to figure out when there is a problem. When one fault code is already set, the computer knows it can't rely on that circuit to provide accurate readings, so any tests that need that data to be compared to, will be suspended. A defect can develop and never be detected. Once you have the first problem fixed, the suspended tests will resume, and THAT is when the next problem(s) will be detected. The Check Engine light may turn right back on. Of course you assume the mechanic didn't diagnose or repair the problem correctly, but in fact there was a second problem he had no way of knowing existed.
Monday, February 13th, 2017 AT 6:26 PM