One problem related to service engine light on has been reported for the 1999 Plymouth Neon. The most recently reported issues are listed below.
The sunroof would only open on right side/ was binding on the right. Had to have sunroof assembly replaced. Noticed oil pooling under my car. Took it in and was told it was the head gasket leaking oil. Had it replaced. 12 days later the service engine light came on and took it back to the dealership. They said there were deposits on the valves and the springs had to be replaced.