Check Engine light problem

Hi there,
I'm new to this forum and I need some help.
I own a 1993 Subaru Impreza Wrx Ra
Recently the Check Engine light has come on and won't turn off. I've checked oil and water. It used to come on when the engine was cold and turn off when the temperature got up to half way. Now it just stays on the whole time. There is no difference in the driving of the car so if someone cold help me out it would be great
cheers

did you get gas recently...if you don't tighted the cap enough it will set off the check engine light.

yeah i did that once an i couldnt figure out why the check engine light came on, i checked the book and it said that could be a reason, so i went and tighten it, and it only took 5 days for the damn thing to go off lol. now i make sure i tighten that thing real good :-)

Thanks for the help, I did a diagnostic check and it turns out it was the Water temp sensor. so I'm getting that replaced. That should do the trick