Are you sure you were not seeing the "Low Oil Pressure" warning light?
You are repeating a common misconception about the Check Engine light, even among many mechanics. They still think fault codes can only be read or are only there while the warning light is on. If that were true, what would be the point of having fault codes? They are instrumental when figuring out which circuit to diagnose for the cause of intermittent problems. How are you going to know where to look for the cause of a problem that acts up once a month for only a minute or two? It is the codes that are stored in memory that provide that information.
Also, be aware there are over 2,000 potential diagnostic fault codes, just for the Engine Computer. Only about half of them refer to things that could adversely affect emissions. Those are the codes that turn on the Check Engine light. The light never turns on for the other codes.
Wednesday, February 15th, 2017 AT 7:39 PM