Service Engine soon light Stays On. Gas cap solution worked once.

What is your question?

The Service Engine Soon light is on and stays on. We tried removing and putting gas cap back on, which worked for about 3 weeks, now it's on and that method doesn't work. High mileage, periodically add oil though not much. No leaks..also need to add coolant to overflow about once yearly, again, no leaks. The vehicle runs fine, temperature is always fine. For it's age and mileage the vehicle runs great!