2005 300C multiple codes

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09-25-2018, 06:39 PM

Alcarl88

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Join Date: Sep 2018

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2005 300C multiple codes

1st post. Trying to help my son.

He was pulling out of his apartment and the car died and would not crank again.

I came over and put my scan tool on it and got 3 communication codes.

U0100
U0103
U0121

From what I’ve found that’s a loss of communication to the TCM, ABS and Gear shift module all at the same time. It seems odd that they would all come up together and so I’m leaning towards it all being some common problem.

I couldn’t find any bad fuses and the battery should be good (plus I had the cables on it from my truck).

Anyone have a place for me to start looking? I’ve been working on cars since I was a kid but when the computers get involved I’m a little out of my element.

Thanks for reading.

10-11-2018, 01:44 PM

Alcarl88

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Well, I'll update just in case this helps anyone else.

I'm not familiar with this car at all (like I said, it's my son's) so I didn't know there was a second fuse box. Turns out there was a blown fuse there (number 29). $120 is expensive for a fuse but I was expecting something far worse when I dropped it off at the shop.

10-16-2018, 06:34 PM

2005 300 LIMITED

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A 5 Amp fuse costs $3.49 at AutoZone.

What else did the shop do for $120???

Last edited by 2005 300 LIMITED; 10-16-2018 at 07:26 PM.

10-17-2018, 07:18 AM

Alcarl88

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Join Date: Sep 2018

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That was about an hour and a half of trying to find the problem. Initial thought was broken wire or maybe a different module that failed and was taking up more than its share of the communication loop.