I get many questions about the "check engine" light.  Here are the basics to remember.

If the check engine light comes on, and then goes off, and if this happens now and then without much reason or correlation to why it is coming on and off, its not much to worry about.  Most likely it is an oxygen sensor that is becoming insensitive.  Sometimes, putting a product called "SeaFoam" into the fuel tank will clean the oxygen sensor and it will start working again.

If the check engine light says on constantly, but your fuel mileage has not dropped or gone up, you can wait a while before you have it checked out.  You should always be checking your fuel mileage.  If the fuel mileage has dropped, get it checked when you are able.

If the check engine light is flashing, get it checked out as soon as you can.  Most likely a spark plug is not firing and raw fuel is being dumped into the exhaust system and an explosion possibly could damage the exhaust system.