this is what ive learned with my ongoing problem. Ive had my CEL on for about 2 years now and the code is PO421 which is a catalytic converter problem. since i knew what it was i never changed it cause a new cat cost about $500. then i went turbo so there was no need for a cat but then i got pulled over and since i live in california, i have to remove everything, put all the stock parts on and go to a smog referee. since my cat is bad the check engine light is still on but i knew my car would pass smog anyways, so i decided to make the CEL go away by unplugging the battery before going to the referee. i passed the smog test, but i failed the computer test because i have a code pending which is due the cat. so... i went to my friend who works at a smog place and hooked up my car to their computer. he explained to me that when you do have a CEL and you make it go away by reseting the ECU or unplugging the battery, it is still stored on the computer until the car clears it. making it go away using the ODBII scanner works, but you will still have a pending code because the ECU must run with the car for a certain amount of time and clear itself. apparently there are a bunch of sensors hooked up to the ECU and they all must be in status "ready" to show they are clear. to finish my story, i ended up buying a new pre-cat but luckily i found 1 for $226 instead of $500 something on convertergeek.com. hope this answers your CEL question.
