because there are hundreds of "codes" that the ECU uses to trigger a CEL when what is measured is compared to what the value should be and the measured value is out of range. Generally Porsche diagnostic books contain a few word explanation of the code and a list of the possible conditions which can trigger that code. As well as a flow chart explaining the diagnostic steps.
So the first step in diagnosing a CEL is to understand the exact code (or codes) that lie behind the display. The good news is the CEL on solid is not the most serious kind, the kind where you stop the engine and have the car immediately flatbeded to the mechanic. But even a solid-on CEL is not to be ignored until its seriousness is thoroughly understood.