Hopped in my car one night to head home. When I shifted to reverse, the car jerked pretty badly and the check engine and AT light came on. None of the lights indicating gears would work, and I had to drive home at a slow 30 MPH. After some research, I found out it was due to a faulty transmission control module. A brand new one is around $450-500 dollars. I purchased it and was set of fixing it without paying a mechanic (there are videos and tutorials on the internet that detail the process).

It took me several hours to fix. Normally, this should take at most an hour or two, but this is my first time attempting to fix a vehicle. It was a pretty easy repair until I had to remove the last nut that held the TCM down. It's located in a terrible location and it is very difficult to remove with a wrench or socket wrench. I ended up using a sliding t-bar to get it off. The video I attached shows the nut that I'm talking about.

After removing the nut, putting in the new TCM and putting everything back took me only 15-20 minutes.

If you don't have the proper tools or the experience, prepare yourself because you'll end up spending the entire day trying to replace this damn thing.

Tools I used:

10mm & 17mm open-end wrenches, socket wrenches Sliding t-bar/breaker bar Impact wrench

The plate underneath the battery box and directly above the TCM uses two 17mm bolts and were screwed on extremely tight, thus the need for an impact wrench.

While driving normally, the car downshifted with a jolt. The AT light blinked briefly but the car continued OK. However, this repeated again and the AT light stayed on. The check engine light came on as well. Now, the AT light stays on and transmission shifts with a jolt to reverse and to direct.

The OBD code was U0101 indicating communication failure with the TCM (transmission control module). Found several other threads on online forums indicating that the problem is with the TCM. Took the car to a transmission repair place for diagnostics. They didn't find anything wrong with the wiring, so the next step was the TCM. However, given the problems reported online, the mechanic said the dealership would be best to handle this kind of problem.

I ordered the TCM myself (part # L32E189E1D, supersedes L32E189E1B) for $525 shipped. Will update this thread after I replace the TCM.

Update from Feb 19, 2014: Received the transmission control module (genuine Mazda part) and replaced it myself. Have driven it for a couple of miles. So far, no problems. Shifting is not smooth at times, but it is expected with a new module while it calibrates. Will provide another update after a few weeks or earlier if the problem occurs again.

Update from May 4, 2015: The new TCM module recalibrated within a week or so and transmission started shifting very smoothly and predictably, probably even better than before the symptoms occurred. No issues a year and about 10k miles later.