"Check Engine" Light v. "Maintenance Required" Light

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The Maintenance Light is a service interval light indicated by the manufacturer. If light stays on solid, it means I have hit the mileage limit for any problem and I need to get the car fixed now. The code will tell the mechanic what specific problem needs to be fixed.  I just now read that when the "Maintenance Required" light comes on, it means the following:

"Usually this means you need engine oil or a scheduled maintenance, in other words, an oil change or a tune-up.  Try the small things first, like adding engine oil or transmission oil or break fluid or even window washer fluid.  If this does not turn off the light, then see a local and trusted AAA Certified Mechanic."

Check engine light is more important for vehicle longevity. Drive-line related. Emissions related. Environmental related. No specific thing causes the light to come on because that light refers to a system within the engine. Solid light means that there's a sensor that has failed or that the sensor isn't working. Air/fuel ratio sensors. Oxygen sensor. Gas-cap sensor. Intake valve leaks. Fuel ratio sensor. Catalytic converter sensor.  One mechanic did suggest to me that the EGR valve light could have been the result of the gas cap having not been sealed properly.

If for any reason you need to, here are the instructions for changing the oxygen sensor near your catalytic converter.