2007 Toyota Corolla Engine warning light

My engine warning light came on and is staying on. After checking the manual, it stated that the light sometimes comes on if the gas cap did not click. I am not sure when the warning light first came on, but I do remember that the last time I gassed up, the cap did not click. But it was freezing and I didn't feel like messing with it! After noticing the light, and reading the manual, I reopened the tank and forced the cap to click. It didn't stop the dash warning light. I checked the oil, brake fluid, steering fluid, and coolant. My car is making no noises, starts well, and runs smoothly. The only thing out of the ordinary since it came on is that it has been very cold. Any ideas what is going on?