One problem related to gas recirculation valve (egr valve) has been reported for the 1999 Plymouth Neon. The most recently reported issues are listed below.
My Neon has 86,000. I have had it from new. In November of 2003, I had my second head gasket replaced. I believe I am on my way to the 3rd. I have an appt with the dealer tomorrow. I have had the egr valve replaced and my sunroof has not worked for the last 4. I had it fixed 4 years ago only to have it malfunction within 4 months. The dealer stated it was not under my warranty, even though I have the extended warranty. The last dealer admitted there was a problem with the head gasket from factory and Chrysler changed them. Apparently not since my head gasket went again in less than 18000 miles. 3 head gaskets in 6 years.