Just found this:

U0121 Lost Communication With Anti-lock Brake Module

How do I get them back together?

 

U0121 is lost communication with the ABS module.

Sorry it sure sounds like you've fried the TIPM. Seen that before adding lights ( more load ) to the TIPM directly.

Sorry for the bad news.

Marco

 

When Monitored:
With the ignition on
Battery voltage between 10 and 16 volts
IOD fuse installed
TIPM is configured correctly
Set Condition:
Bus messages not received from the Antilock Brake Module for approximately 2 to 5 seconds.

Possible Causes
CAN B OR CAN C BUS CIRCUITS OPEN OR SHORTED
DTCS RELATED TO BATTERY VOLTAGE, IGNITION, OR VIN MESSAGES
TIPM NOT CONFIGURED CORRECTLY
ANTILOCK BRAKE MODULE POWER AND GROUND
ANTILOCK BRAKE MODULE
MODULE THAT SET THIS DTC


Diagnostic Test
1. VERIFY DTC IS ACTIVE

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NOTE: Ensure the IOD fuse is installed and battery voltage is between 10 and 16 volts before proceeding.

With the scan tool, read active DTCs.

Is this DTC active?

Yes
Go To 2

No
Refer to the Stored Lost Communication test procedure. Refer to the table of contents in this section.



2. CHECK FOR ANY OF THE FOLLOWING ACTIVE DTCS

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With the scan tool, read all active DTCs from all modules.

NOTE: Check for TIPM configuration, CAN B or C hardware electrical, VIN Missing/Mismatch, battery or ignition related DTCs.



Does the scan tool display any active DTCs to the conditions listed above?

Yes
Diagnose and repair the DTC. Refer to the Index for a complete list of the symptoms.

No
Go To 3



3. VERIFY THAT THE ABS IS ACTIVE ON THE BUS

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Turn the ignition on.
With the scan tool, select Network Diagnostics.
Verify that the ABS is active on the bus.

Is the ABS active on the bus?

Yes
Go To 4

No
Refer to the No Response test procedure. Refer to the table of contents in this section.



4. CHECK FOR ADDITIONAL COMMUNICATION RELATED DTCS

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With the scan tool, select Network Diagnostics.

Is there more than one module with active DTCs “Logged Against” the ABS?

Yes
Replace/update the Antilock Brake Module in accordance with the service information.
Perform ABS VERIFICATION TEST - VER 1.(Refer to 5 - BRAKES - DIAGNOSIS AND TESTING)

No
Replace/update the module that set this DTC in accordance with the service information
Perform the appropriate VERIFICATION TEST.

 

Marco.....can the Smarty check vin in the ecm?

 

I found this by searching U0121 :


[QUOTE] "Dodge must have put a 67 liter fuse cover on my 07 because it said 30 was a spare and sure enough the dinks that put it together only put like a 7.5 fuse in and i never checked it because i thought it was a darn spare. Thanks for all the help... i REALLY appreciate it, you guys are awesome. Thanks agian"[QUOTE]

I will try this seeing as I use a 'spare' to fix the one that had blown. If not, vzdude, I don't have a scan tool to check the 'Network Diagnostics'.

 

Yeah, I figured you probably didn't. I have seen some ECM's lose the vin programming after a flash update. Not real often, but for some reason, intermittently, one will do it. Low battery voltage or something? Can't say for sure. May be as simple as paying the dealer for a little of their time to double check it. Typically, what I've seen, is once the ecm loses the vin #, it doesn't know to look for the ABS module or won't because of the lack of vin.

 

It's not a fuse situation that I can tell. I know it wasn't like this with the 4.11 download. It happened after I updated the truck back to stock. I, in turn, put the 4.11 update back in & it's still there & won't go away. It erases it but it instantly comes back almost like it doesn't erase it. Any other suggestions are welcome. Do ya'll think it will mess anything else up driving it like this till I can get it to the dealer?

 

vzdude,
WHENEVER an ECM in these 5.9L CR trucks is programmed with the StarScan or DRB the VIN number becomes erased from the ECM. ALWAYS. One needs to program the VIN and skimm code manualy. Smarty does everything automatically.

Of course it is possible to check the VIN# with Smarty. Press key # 5, keep it pressed plug in Smarty, keep pressed for another couple seconds. Now navigate to the ECM update menu. Press # 1 = Ecm update. Instead of updating the ECM Smarty will display the SW revision number and then pressing the > key the VIN number from the ECM. The VIN # scrolls and can be difficult to read, press key # 0 then the VIN # will pause as long as key # 0 is being pushed.

Marco

 

Maybe it depends on the flash, because I've flashed numerous trucks and only had to reprogram vin a couple of times, mine included. What can the smarty do in this instance? Is it capable of reprogramming the vin in this case, if in fact that is the trouble?

 

vzdude, on the lid for my fuse panel under the hood is says fuse #17 is a spare but it has metal tongs in there unlike the other 'spare' fuse locations. This is the fuse I pulled out to replace the one that blew(both 15's). I read on a dodge forum for 300 C's & they purposely remove this #17 fuse so they can drive it harder but sometimes it will throw the code U0121! They just put the fuse back in, erase the code & all is normal. I have replaced it & tried clearing the code but no dice, it comes right back. If it got stored in my 4.11 program then I went back to stock, would it store it in the 'stock' program as something that is supposed to be there? I just put it back to stock & disconnected both batteries to see if it helps, which I doubt.

 

Try to read the vin in the ECM like Marco described above. If it's not there, you may have to have the dealer reprogram the vin. That's kind of what it sounds like.

 

On second thought. IF the VIN# was erased (or changed a single digit) Smarty would no longer work on the truck and say VIN locked... Means since there's been another update that the VIN is in the ECM.

 

O.K. ! After RE-READING the first post........you should be fine. There are times, like when flash updating, ( which is how the Smarty works if I understand correctly ) that the CAN is shut off. That would be why the communication would be lost and set that code. Everything should be as normal and the code will eventually go away with enough key starts. As far as the lights go , if you still want to add them, either add a relay and use the current light wire as the trigger for the relay to run your additional lights, or have the dealer update the TIPM to handle the additional load. I was under the impression that there was "something" still not working correctly on the truck or you had some warning light on. Unless I'm reading something wrong, you're fine! Drive it!

 

The speedo, odometer, & overhead are NOT working. The ABS & parking brake lights stay on.

 

Hmmm.... running out of possibilities.......do you have the hands free module - UConnect? Have you unhooked the batteries for a little while? Sometimes that can reset a confused module.