U0101 lost communication with tcm

AdamLVWAdamLVW Member Posts: 1
edited January 2019 in Mazda
I have a 2010 Mazda 3 that recently showed a simultaneous check engine light and AT light. It was making a loud clunk sound when shifting into reverse and was stuck in what seemed to be 3rd gear (limp mode). The code is in the title. I checked the TCM visually and could not find any obvious damaged components. I had to peel off the resin to get to the board. So then I resealed it before putting it back in the car. Instead of replacing the TCM immediately, I plugged it back in and waited. The warning lights came back after a day or so but then the AT light turned off. It’s been intermittently going into limp mode and then back into regular shifting mode. The check engine light is on most of the time but AT light is off now. However the U0101 code persists. I am confused because if the TCM were not working why is it not in limp mode all the time? I bought a multimeter and checked the female connector to the TCM and discovered that pin letter I (as in iodine) was not receiving battery voltage with the key on. It was zero voltage. I then checked the wire going into that pin and it also had no voltage. Plus, pins 6 and 14 on the OSB2 read about 100 ohms, even though they are supposed to be 60. I checked the TCM fuse and AT main relay in the engine fuse box and they both were intact and working. If the wiring is broken somewhere I have no idea how to find it because most of the wiring goes into nomans land deep under the parts. I looked up wiring diagrams but I’m a bit lost regarding how to find the source of the lost power.


  • PF_FlyerPF_Flyer Pennsylvania Furnace, PAMember Posts: 9,400
    Can you find the other end of the wire that has no power from the wiring diagram? You could then check for continuity of the wire. At least if you have where it starts and stops, you might be able to deduce where the wire must be. I'd also check for voltage at that other end. If none, then you know the break is further back along the path.
Sign In or Register to comment.