I have a 2000 and cant get the check engine light to go out. I have code P0508 and of course 1684, I have replaced the iac,cleaned the throttle body, check the air filter, did a continuity test on the four wire to the pcm, replace the pcm, replaced the pcv valve, inspected throttle body boot for cracks and correct fit, checked the vacuum line to intake manifold, and checked the gaskets on the intake manifold.Just replaced one 02 sensor and cleaned the other. still no luck, Can anyone give me any ideas on what i should check next.

I did notice that when the car is at idle the throttle boot does seems to be slightly collapsed but doesnt cause restriction, i dont know if this normal for neon or not, thats what lead me to do all the checks on the intake side as previous listed

 

If at idle the boot seems slightly collapsed at thottle it will probably collapse fully. I don't know if this would cause your code but I do know its not a good thing.