Excell Check Engine Light
 

I have a 2000 Hyundai Excell, 1.5 Litre DOHC 16v. I live in Australia, but it looks like ones they sold in the USA mid to late 90s. Anyway, nothing wrong, the thing drives fine. Runs great. The Check engine light comes on now an again. Generally on a hot day, sitting at light for longer than normal. The temp gauge is not even close to being hot. It has been doing this for almost 2 years. (a car I only commute to work in, don't drive otherwise.) So wondering what some possible problems would be.

The engine does have a thermostat to keep the engine in the best operating range. Thermostats can stick open and the engine never reaches operating temp. This sometimes can be observed when the temp takes a long time to warm up.
When thermostats sick closed the coolant can't circulate through the radiator and the engine overheats.

The car has a functioning temp gauge. The temp doesn't show too hot. Also, this reminds me. If the light is on when I turn off the car, it stays on. So when I start car again it is on. Goes away shortly after. I guess it COULD be sticking when it gets hot, but doesn't seem to be the problem. Thanks for the info though, will look into it.