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Q:

cycles start/stop when switch first hits Run position on 2003 Chevrolet Silverado 2500 HD

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03 Silver. 2500HD 8.1. instantly cycles start/stop at 3 second intervals when key initially hits Run (not Start), but doesn't start, just cycles. Can start and run by pushing key hard past Run to Start, then it starts. Problem started after dead battery tries, and new batterey installed.
(1) Answer
It sounds like there's a problem with the ignition switch and lock assembly but I can't say I understand the question exactly. Tow it, or drive it if it runs, to a GM specialist or dealer, there have been some problems with the anti-theft systems in these vehicles, but I don't think that's what you're describing.
If you want to find a specialist, here's a directory link for you:
https://repairpal.com/directory?address=97056&car_brand_names=Chevrolet