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Discussion Starter #1
I picked up 2015 Silverado crew cab 1500 z71 ltz. I swapped out the halogen bulbs for some led bulbs. The next morning I go go start the truck and it won't start. I have power, the gauges light up, but when I go to start it everything goes dead but comes back on when I release the key. I put the original bulbs back thinking it was the led bulbs and no change. Then it started without a problem about an hour later. Battery is 1 year old and measures 12.4V. No codes stored in the ecu. I use Blue Driver code reader which scans all systems, not just emissions codes.

Something I noticed that doesn't make sense to me. When I turn the key to the on position before I start it, the check engine light is NOT on and the truck won't start. If I cycle the key for a few minutes and get the check engine light to come on when the ignition is on, the truck will start without a problem. Is there a security feature or theft lock that is preventing my truck from starting? I thought those led bulbs messed something up but this has happened to me 4 times in the last 2 days and I don't know what system to check. There is definitely some kind of correlation between the check engine light being off and the truck not starting. Why is it intermittent? I'm going to check all of the grounds tomorrow but other than that I'm stumped. Thanks in advance
 

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I am thinking that the engine should start immediately even if check engine light is on briefly. In fact while starting a variety of dash trouble lights should come on and then go off. So if the check engine light is not lighting maybe the the power required to light the check engine light do the test routine and also to start the engine is all missing. If you have power to run the test light then you have power to start the engine. Having said that not sure where to start looking though. An intermittent problem is the worst case to investigate. But sounds like a loose connection somewhere. A ground is a good guess as you have deduced already. once I had a loose connection in a fusible link style fuse right off the battery. The fusible link was good but one end inside the connector was semi burned and caused an intermittent. Took a week to find it.
 

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Intermittently my Sierra won't catch when I hit start. I've cleaned every ground on the truck but the one buried behind the dash board. I suspect that's what's making my gauges go goofy as well, perseverance the word here
 

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Discussion Starter #4
Update: My center display would say service 4wd when the truck doesn't start. I didn't think it was relevant at the time, but this morning I went to start the truck and had no problem. It started right up, and did not say service 4wd. Could a faulty 4wd circuit cause my truck not to start?
 

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Discussion Starter #5
2nd update: More strange revelations...

If I use the key to start the truck, it's hit or miss just as I have described above. If I remote start it, it starts everytime (so far) since yesterday and today. Several starts all day both days using the remote start. Tonight I put the key in it and it's back to not starting. Also, once it doesn't start with the key, the remote start will not start it either. Wtf
 

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Can you check for codes? Does the drive system show up as a fault? Anyway to check the anti theft? If the remote works means the anti theft is happy with the remote but not always happy with the key, maybe. How about the ignition mechanism there are some mechanical bit that may be checking the key for anti theft compatibility. Just thinking out loud here, sorry. Regards Rob
 

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Discussion Starter #7
Codes!
Screenshot_20210403-091912_BlueDriver.jpg

I never mentioned the codes for the transfer case or front diff bc I didn't think they were relevant, but the bcm code worries me a little
 

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I wonder if the BCM is playing tricks with the key detection circuits as the remote will work. Also it could be playing with drive train detection codes. Still not sure how to check the BCM however.
 
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