how serious is the 'check engine' lite?

[Perfectlap]

CEL can go off for the most minor thing like not screwing on the gas cap all the way (at least Three clicks). the light will not go away by itself, you need an OBDII scanner to erase the code and turn off the light.
What I do is to keep an OBD II scanner in the rear shelf. I picked it up on Ebay for about $100. Plug it in, read the error code number and do a quick search on Renntech.org to see what kind of problem it is. I thought of printing out all the codes and keeping that with the scanner.