I was coming back from a job interview today, and being that I don't get on freeways that often, I decided to let the Max do its' thing. Well I was doing about 90-95 and everything seemed fine. After I took my exit I noticed the dreaded CEL light was on. Upon getting home I checked to see what code it was and I got a "0504 - Automatic Transmission Control Unit to ECU". Now you may be about to ask - did you notice anything strange? Gears not shifting correctly, etc? No, everything seemed fine to me.. I also got a 0104, which happens to be the speed sensor I JUST replaced, but that's another story. I'm a lot more concerned with my transmission than my speedometer. Anyone know what would cause this 0504, and what would be the appropriate fix for it? Thanks in advance guys.

 

From Stickys

0504

Diagnostic Trouble Code 0504 points to a problem with the Automatic Transmission Communications line. Pulse signals are exchanged between the Engine Control Module and the Transmission Control Module to assure smooth shifting during hard acceleration or deceleration.

This malfunction is detected when the ECM continuously receives an incorrect voltage from the TCM. Possible causes include...
- Harness or connectors (The communications line circuit between the ECM and the TCM is open or shorted.)
- TCM
- Discharged or faulty battery

 

I went through the same exact thing, 0104 + 0504 even after replacing a VSS.

If you don't see any noticeable problem in speedometer or cruise control, try taking out the gauge cluster and re-tighten screws, especially speedometer. It may sound ridiculously simple but it fixed in my case and I haven't got either 0104 or 0504 ever since, for more than a year. If this doesn't fix your problem, it may be wiring problem between VSS and ECU (I tried this first but it wasn't successful).

I read that the other .org member had to replace whole cluster to fix the similar problem.