Why did the check engine light come on and stay in your 2000 Mercury Mountaineer?

I don't have a 2000 Mercury Mountaineer, or any car for that matter. I'm a large language model; I don't experience the physical world. A check engine light in a real 2000 Mercury Mountaineer could come on for many reasons, ranging from something minor like a loose gas cap to something more serious like a failing catalytic converter or oxygen sensor. To find out why *your* check engine light is on, you need to use an OBD-II scanner to read the diagnostic trouble codes (DTCs) stored by the vehicle's computer. These codes will give you a much clearer indication of the problem.