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Discussion Starter · #1 ·
Mazda 3 2010 GT 2.5

Hello all!
I was hoping someone here might be able to help out since we (the mechanic, myself and dealership also) haven’t found a solution to the problems and are going in circles.

One week ago, my car suddenly showed the AT and traction control sign after leaving a grocery store and it would not go away. The car did not detect any gears and would “jump” when moving to the Reverse gear or Drive (the jump on Reverse is bigger than Drive). After that, the audio player was able to play the audio, but the volume change or seeking songs or switching modes from Bluetooth to radio was intermittent. It did not work most times, but it would work for the first few seconds when I turned off and then turned on the car. The audio controls on the steering did not work too – I was able to use other steering wheel controls like accepting/rejecting calls, changing info display, cruise control.

Here were the codes my ODB reader returned: U0101 for transmission issue.
Old codes: P0138, P0421

The mechanic then checked the fuses yesterday, also the sensor and wiring and did not find anything wrong. He said he simply reset the computer and things were normal. I turned on the car at the garage and took it for a test drive, all was okay. After coming home and trying again after some time, the errors had returned. But this time, the heated seat on the bottom area(butt) is not functioning but the backrest is working fine. The audio system does not respond at all anymore (it works in the mode it was in, but can’t increase or reduce volume, change track or move from Bluetooth to radio).

And today when I drove, the car looked normal but after say 30 minutes or so, there was a momentary jerk on the road and the AT light came on for 1-2 seconds and then disappeared again and the car was in “D” again.
The mechanic now wants to try to change the transmission sensor. What could the issues be here, and what should my next step be?

Thank you!

· Registered
235 Posts
Well here's some thoughts:

P0138 - probably oxygen sensor issue, if wiring is OK, possibly bad O2 sensor, you can see if the sensor is outputting any voltage or maybe the heater is dead and that throws a flag. Read this:

Check engine light - P0138 | 2004 to 2020 Mazda 3 Forum and Mazdaspeed 3 Forums (

P0421 - Not sure, lots of info on Google about this code

U0101 - TCM not communicating, I had this, had to replace TCM (transmission control module). Rebuilts are available, but in my case, I had to get a new unit.
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