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'05 Neon SXT issue(s)

I need help. So I bought a 2005 Neon SXT for my wife with 118k miles on it. Background to problem: We had a high idling problem that was resolved (kinda) by replacing the TPS unit. Also had the timing belt and water pump replaced (since I had no idea when it was last done), and as I was driving home from the mechanic’s the check engine light came one. OBD-II code was P0507. I cleared the code and replaced the Idle Air Control sensor…
Which brings me to the current problem: right after I replaced the IAC, whenever I go to start the car, the electrical in the car goes completely out (no dash, no odometer, no door lock, nada). If I let the car sit for a while, the odometer comes back on, and if I try to start it again the same thing happens. The only sound it makes is a single click as the power goes out, and I’ve tried starting it with both the old and new IAC units. I am totally confused what might be causing what appears to be a temporary short, so any ideas would be greatly appreiciated.
Thanks in advance for any suggestions.

Since you unplugged the iac, I would first take a close look at the connectors. Make sure no wires came loose and make contact with each other or ground. If that don’t help, put the old iac back in and see if that makes a difference.

The battery terminals are dirty or loose.

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The electrical weirdness on attempted cranking is likely some kind of battery problem, either cable connections or the battery itself. Commonly reported here this time of the year b/c of colder temperatures. The IAC code may well mean the high idle problem is (or was if now replaced) a result of the idle air control gadget. The computer programs the IAC to set the idle speed for the conditions (engine coolant temperature, alternator load, etc), and if the computer doesn’t see the engine rpm doing what it programmed it to do it will throw an IAC code.

IFf cleaning terminals don’t work, clean cables at starter and ground, then check cables. Probably won’t need to do anything past terminals though.

Thanks for all of the input. I checked all of the wires and connections and no faults found. On a whim, I disconnected the battery cables from the terminals and held them together for about twenty seconds to make sure the computer memory was cleared (in case there was stuck in a loopof some sort) and sure enough it started right up.Everything is running fine now except after it warms up it climbs up to 1100-1200 RPMs at idle in park, but idles at 800-900 in gear (its an automatic). At least it runs, but just would like it to run the way it should.

I expect what happened is upon disconnecting and re-connecting the battery you now have improved the electrical connections to the battery. Lower resistance connections than you had before in other words. That could happen simply by installing the connector on the post in a slightly different orientation, or perhaps the connector/post connection is tighter than before. Another possibility is the cables are frayed inside of the insulation (so you can’t see the problem) and repositioning the cables has temporarily fixed it. If the latter, may work for a while, but it won’t last. I expect however it is the former, and you are good to go. Best of luck.

It seems that you still have a vacuum leak, the TPS and idle air control motor were not the problem.

Check the intake hose from the throttle body to the intake manifold for leaks, make sure the clamps are tight.

P0507, Idle speed higher than target speed can also be caused by a broken PCV valve allowing too much air flow.

Nevada_545 (and anyone else),
Would a bad PVC valve cause it to idle normally in gear, but higher (@1100-1200) in Park at operating temp? Also, would that be the cause of a P0507 code? I have also been told that it might be either an exhaust gasket/manifold leak or an o2 sensor. If its an o2 sensor, would it be the upstream one?
Thanks for any and all advice/input/help.

UPFATE 3: Fixed. So here’s what happened - Took the car to a mechanic who ($549.00 later) replaced the TPS and IAC (again), and also said that the connector to the IAC still needed to be replaced, and there was no change to the condition, BUT he was also not able to get the code to show back up on his scanner after three test drives. He also pointed out that the airbox had a Pick 'n Pull marking, indicating that it was not the original airbox. He also said that he thought it was the computer that was bad, since it had already been replace a year after the car was built (he showed me the lable on it). So as I’m heading back to said mechanic to pay the deposit on a new computer, I decide to ask my friend who is knowledgeable about such things, and at first he is baffled until I mention the airbox. He reaches under where the throttle/cruise control cables go into the throttle body/airbox and suddenly the car idles normally. He then takes the airbox out so he can better access the connector, and turns an adjustment screw a half turn and the car if running normally. He (my friend) said that the airbox was probably set for either another car or someone else had tried making an adjustment, and that the other mechanic didn’t think ‘outside the box’ (his words, not mine). Our SXT is now running like our '03 did when it was new. Thanks to everyone for all of the help and suggestions.

I have no idea what the screw does, but it sounds like adjusting it worked. The two common things that are adjustable in that area on some cars are the TPS and an idle rpm bypass screw. But I doubt your car – being an 05 – doubt it has either of those adjustments. In any event glad you are back on the road with a smooth running engine. That screw may just a place to tighten a clamp for air hose to the air box housing, so it doesn’t leak.