This is a 2001 Saturn SL1 Coupe 1.9L 4 cyl automatic with 100,900 miles. This vehicle has been serviced regularly. I am not sure if a tune up wasdone recently. The problem is the car will start fine for a while and then not start again. The engine begins to turn over but then stops. We have had the car checked out by our mechanic who says there are old codes in the system but nothing that would cause it to not start. He is believing it is a sensor on the motor that tells the computer when to allow the vehicle to start. Is this possible or could there be another problem? If you leave the vehicle sit for several hours or more it will start again but then you don't know when it will not start. The last time we had the vehicle towed into our mechanic it sat overnight and the next day it started. Then it quit again and he checked for codes but none were displayed. Also the service engine soon light stays on as does the service light with the little wrench. Anyone have any ideas this car is again sitting in my driveway deciding not to start again. Thanks for all the help.