check engine light off by itself

I don't know what kind of vehicle you have, but if it is OBD II compliant, most diagnostic trouble codes have 2-trip fault detection logic. If the fault is detected for 2 consecutive trips, then the malfunction indicator light (MIL) is illuminated, and conversely, and if the fault is not detected on 2 consecutive trips, the MIL light is not illuminated. There are specific parameters that define a "trip" for each vehicle, like driving at a certain speeds for a certain amount of time, etc..

Most likely, the condition that was detected is intermittent and is not always detectable.

2001 Toyota Corolla. Interesting it is on for a few days and now off. Oh well, I will take your advice and think it "fixed" itself.

All vehicles sold in the U.S. starting with all 1996 model year vehicles are OBD II compliant; some earlier vehicles are as well. The MIL, A.K.A. check engine light, will probably come back on with the same trouble codes as the problem gets worse and more consistent.

I've seen quite a few posts regarding catalytic converters and O2 sensor problems with that vintage Corolla. Major emissions components like the catalytic converter (but not O2 sensors) are covered for 8 years or 80,000 miles, whichever comes first, from the date of first use so if your Corolla falls within that range, I'd take it to a Toyota dealer if the light comes back on with a converter fault. California and Massachusetts have enhanced emissions coverage for evaporative emissions control devices so check the faq section on Toyota's web site if one of those things has a problem.