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Will not start and code U0100

Tiny
STEF0711
  • MEMBER
  • 2006 CADILLAC STS
  • 3.6L
  • V6
  • 2WD
  • AUTOMATIC
  • 1 MILES
Okay, so I recently bought this car used off of someone else. Right away I noticed that the engine wire harness was all jacked up. I do not know what the guy was thinking but he had done a hack job on the harness trying to connect and repair it with those small plastic electrical crimp connectors. So I went ahead and bought another one used also off of eBay and replaced it myself. I also had to purchase a new fob which I programmed myself, but now when I try and start the car all the lights on the dic message center and all that area go blank and shut off for a few seconds. All the while as I am pressing the start but I hear clicking under the hood from the fuse box. I want to say and after a few seconds everything turns back on in the dic message center with a few messages telling me to have this and that serviced. Also I might add when I put a obd reader on the car it give me the UO100 message and another UO100 but with a small pd after it if there's any thing I can please help me with or any ideas. Please let me know. Thank you
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Wednesday, November 20th, 2019 AT 5:46 AM

11 Replies

Tiny
MOTOR MASTER
  • MECHANIC
  • 279 POSTS
Hello my name is Dave.

That code indicates no communication with the ECU, considering what you have had to do to try and get this car straightened out it will most likely take a lot of troubleshooting to track down the issue. The first question that comes to my mind is if the wiring harness is the correct one. But lets assume it is the correct one for the moment, first thing then to check is all the fuses and relays. I am going to attach the diagrams for the power-train so you can compare yours to it. I would start at the ECU and compare the wire colors to the ones in the diagram, if any are different then the harness is incorrect. This will also give you the information you would need to check that each sensor has the correct wires going to them. The other thing I would check very closely are all the ground wires as they are the easiest ones to overlook when changing a harness. Please keep us up to date on your progress and thank you for using 2CarPros!
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Thursday, November 21st, 2019 AT 4:09 AM
Tiny
STEF0711
  • MEMBER
  • 8 POSTS
Well, i'm pretty sure it's the correct harness. I made sure by closely comparing the one I bought to the one I had on the car even the color of the wires and the connectors and rubber seals on the connectors and it all fit perfectly. That being said I'm not sure if I have all the ground wires connected right do you happen to have a diagram that I can use as a reference? And thanks for the the ideas on what to do next. Also I took out my starter and tested it and it works perfectly. So I was wondering what about the crank sensor should I maybe check that? Because right now I'm just stuck on the fact that I cant get my car to even try and start even when I try and jump the relay. It used to crank at least before but not anymore. I think I need a new battery in going to go out and buy one today. Again thank you.
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Thursday, November 28th, 2019 AT 12:40 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
It sounds like the battery is low on charge. To be sure here is a guide to help test it:

https://www.2carpros.com/articles/car-battery-load-test

Please run down this guide and report back.
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Sunday, December 1st, 2019 AT 2:20 PM
Tiny
STEF0711
  • MEMBER
  • 8 POSTS
Yea i'm pretty sure I need a new battery so I am going to buy one today and also i'm going to check all the grounds and make sure they're all in the right place. I'll get back with you for sure.
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Monday, December 2nd, 2019 AT 10:05 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
Please do, we want to fix this problem for you.
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Monday, December 2nd, 2019 AT 10:15 AM
Tiny
STEF0711
  • MEMBER
  • 8 POSTS
Do you happen to have a different kind of link or something that I can look on for the different placement of all the grounds like one that has an actual motor on the picture and how many total grounds and where they are?
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Monday, December 2nd, 2019 AT 8:16 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
Here are the diagrams for the grounds for the car and where they are so you can check them out make sure they are clean and tight.

Check out the diagrams (below). Please let us know what you find.
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Wednesday, December 4th, 2019 AT 12:29 PM
Tiny
STEPHANIE RAMIREZ
  • MEMBER
  • 1 POST
Okay, so I finally got a new battery but now when I go to push the button and start the car it says some kinda message about the cars theft system and does nothing. I'm so frustrated at this point feel helpless like my car is never going to work. Anyways any input would be greatly appreciated. Thanks
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Thursday, March 12th, 2020 AT 8:01 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
When you try to crank the engine over does the security light flash? If so lets try to reset the system. here is a guide to help us:

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please run down this guide and report back.
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Thursday, March 12th, 2020 AT 11:00 AM
Tiny
STEF0711
  • MEMBER
  • 8 POSTS
The security light doesn't blink but it just stays on. Since my car uses a push start button instead of key in the ignition how would I follow the procedure to reset the system because none of the instructions are for a push start ignition. So i'm kinda confused by this, lol. If you could please let me know something. Thank you because you have been so helpful with all of this and I really really appreciate all your help!
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Monday, May 4th, 2020 AT 6:26 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,702 POSTS
If it just stays on the security system is not activated. the U0100 code is a CAN issue like one of the computer is not working (usually the PCM) let's do a CAN or ALL scan. This video will show us how.

https://youtu.be/InIlnsjOVFA

Here is how you replace the PCM in case that is the problem after the scan. If you get a rebuilt PCM please give them your VIN so they can program it for you. Check out the diagrams (below). Please let us know what happens.
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Monday, May 4th, 2020 AT 11:50 AM

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