I have a 2001 Chevrolet Cavalier with a service engine soon light on constantly.  You  
can reset the computer and it will go off until you put the car in gear. Then it will cause  
the DLR light to blink and the left headlamp will blink repeatedly for several seconds the
 go off but the service light then comes back on. Any ideas?






JK:
Sounds like you may have a wiring problem to that headlight. This would also cause the
Service Vehicle light to come on.
A common problem on the Cavalier is green / corroded wires in the harness between  
the left and right side headlights.  
You will need to take apart the harness and tug on the wires looking for one or two to  
break.