Dodge Repair: check engine soon, plymouth service, plymouth neon

Question
Hello-

I have 2 closely related questions regarding my '99 Plymouth Neon (Hiline). Last week the "check engine soon" light came on and the car was driving fine with no unusual noises. I brought it to my long-time mechanic who attached some kind of meter to evaluate the problem, and he replaced some kind of emissions thing (sorry I can't be more specific right now) for $80. The "check engine soon" light went out but after a few days of driving the light came on again.

My 2 questions are:
1. If I bring the car to Dodge/Plymouth service, do they have a method for pinpointing and solving the specific problem, or might the result be the same?
2. My mechanic said that in a situation where the car drives fine with no noises and the "check engine soon" light goes on, that generally speaking the problem is very minor. Was he correct in that statement?

Answer
question 1 The dealer has a better shot at figuring out what the issue is because the scanner we use is made just for Dodge,Jeep,Chrysler.

Question 2  He has no idea what could cause the check engine light on  there are about 900 different codes that will turn on the light.
With out the code number he can not say a single thing about what it wrong and the price to which will be required to repair.