Cadillac Repair: 99 Seville Starting Problem, high voltage system, battery cables

Question
Thank you for all of your help in the past but I am stumped with this car. It all started with the car acting like it had a dead battery (clicking) noise.  I replaced the battery - NO Change, I then replaced the starter - NO Change, I then replaced the neutral switch; it worked for a short time however I now have the SERVICE ENGINE SOON light on which I believe is coming from a "IAT" high voltage system code AND the car still at times gives me a clicking sound...........any suggestions?

Answer
Could be corroded battery terminals and or internally corroded battery cables. If fault was resolved for awhile with change on neutral starter switch, Recheck adjustment and wire contacts within the electrical connector.

As to the IAC. What is the fault CODE ?.