U0121 - I'm lost on this one

CT9AIX

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Background on the truck: Been running the same engine/tranny tune for 6ish months with no issues. Cab was last off in March 2015, truck is pretty much never driven in the rain or any weather other than dry/nice weather.

Was driving home from work with no lights on dash and everything running fine as usual. Left from a traffic light and decided to burn the tires off.... did so and once I was done and still rolling around 40ish MPH going back to normal driving.... ABS light & parking brake light appear and the message center says "Check Brake System". I was about 15 miles from home so I kept driving and everything was perfectly normal. Plenty of power, shifted fine.... if you didn't look at the lights on the dash, you would never think anything is wrong.

Get home pull the codes, U0121& P0500. I tried cleaning the VSS in the rear diff and that did nothing, so I put a new one in and that got rid of the P0500. But now I cant get rid of the U0121. Below is a list of things ive tried and still no success. Any insight and suggestions are much appreciated!

- Checked all ABS related fuses
- Removed battery cables for 2 days
- Cleaned all battery terminals
- Took apart/inspected/cleaned/applied dielectric grease to the following items: VSS, both front ABS sensors, both PCM connectors on fire wall, 3 connections above driver side spring bucket, ABS module itself, two sensors on top of tranny
- Both batteries are in good voltage condition
- Reloaded the same engine/tranny file I was running when event took place. Also loaded a totally different engine/tranny file
- Inspected wire harness going to ABS module, VSS and quick look at each front wheel speed sensors
 

CT9AIX

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Sorry, I should have updated this. After trying all the easy "dumb" things, it ended up being the ABS module itself
 

SmokeyFreedom

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Solved mine today. 08 F350 6.4 Had U0121 and P0500 codes, no speedometer, ABS light on, brake light on, hard shifts. Had just been in working on drivers manifold. I had unhooked all disconnects for easy access and ABS main harness was a bitch.

Tried a speed sensor (VSS), no go.

Reseated ABS main plug repeatedly. Nada

Took apart the disconnects on drivers spring tower, cleaned with contact cleaner spray, reseated and pressed them together HARD! Codes/lights gone!! Came right back on a short drive. Reseated and tried them together with big ass zip ties. WINNER WINNER!!


Posting for the next poor sole to have these issue.
 

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