Your check engine light came on in your 1999 Dodge Neon you checked the PCM codes and the one that is flashing in your car is not in your repair book what does code 45 mean?

There is no such code on an OBD2 vehicle which this 1999 Neon is. Each code has a unique letter and four numbers that correspond to a specific fault. The letter can be a B (body), C (chassis), U (network), or P (power train). Practically speaking, just about every code you'll have to deal with will have a P prefix, but the proliferation of electronics means that the others may come up. A code 45 is an OBD1 code which you will not receive from this OBD2 vehicle.