Q&A

 Master

Both lights being on can easily be caused by the failure of one sensor or problem a scan tool is needed to retrieve codes from the computer in your car to see what he problem is. Evaporative emission system and O2 sensor failure are the two most common failures.
If the Check Engine light/Service Engine Soon light illuminates while you are driving, the car's on board computer sees a fault in the engines management system that is causing an excess amount of pollutants to be emitted from your car. This could result in engine damage if left neglected. Some problems that can cause the Check Engine light to stay on may take two "drive cycles' to complete before illuminating the Check Engine Light. The computer sees a problem and looks for an identical set of circumstances to occur to confirm that it has indeed seen a problem before turning on the light. A Check Engine light that continually flashes while you are driving is serious and is a sign that the computer sees a problem that seriously risks damaging the catalytic converter which may result in an expensive repair or even may cause a fire! Diagnostic codes are stored in the vehicle's computer and can be retrieved using a scantool that communicates with the computer in your car. We have in our Repairpal website an encyclopedia with in depth articles about computer, and computer control systems, emission and driveability problems. http://repairpal.com/check-engine-light

 Rookie

Maybe your gas cap wasn't tightened enough and the system lost pressure due to this setting off the warning lights. I had this happen once.