Hi, I bought a used Mazda 3 in October. (as is with a small extended warranty). Within 3 weeks the transmission started acting up.. dropping from 5th to 3rd gear at constant highway speeds. The A/T light would come on when it was acting up, but would not store the code. I took it to the dealer and all they would say is that it needed a new transmission but couldn't tell me why. So, I then took it to a transmisssion shop. Upon instection they determined that the hydraulic system was shot in the car. They rebuilt the transmission. I went to the shop and picked up the car. Within 10 miles of leaving the shop, it was doing the same thing it was and this time threw an engine code. I took it to Autzone and had the code read. It was U0101 which is basically the TCM not communicating with the transmission. Later, the light went out and the car was acting fine. I decided to drive the car around and check it out some some. It acted fine until I used the manual mode. Then A/T light came on, the check engine light came on, and drove erratic. I called the transmission shop and they said that it may need the computer reprogrammed. However, after dropping it off, they read 5 engine codes. I can't remember them all, but one was the TCM not communicating with the transmission, ABS system problems, power steering problems, a CAS problem, and I can't remember the 5th one. Anyone else have this problem? The transmission shop said they are going to call the dealer to see if this is an issue. The problem is I had the car at the dealer, and they couldn't diagnose much of anything. I am out a ton of money, am broke right now, and it is right before Christmas! UGH@ I need help!

 

What year? Mileage?
I think it's an automatic, standard transmissions don't interact with computer.
Other possibilities:
1) Mazda grounds are minimal. Run an extra ground from your negative battery terminal to the following: transmission, engine block, throttle body, car body. This is a must.
THESE OTHERS NOT IN ANY PARTICULAR ORDER
2) Bad electrical/corroded connections
3) Bad voltage, battery, alternator
4) Bad throttle body. The throttle body somehow interacts with transmission.
5) Bad TCM, ECU
6) ABS sensors

 

Also possible that some connection was broken/damaged/not reconnected properly when they worked on the tranny before.

 

Yes it is an automatic transmission with 84,000 miles. I just recently bought it usd. It has been a mess, still no news from the transmission shop. I called 3 dealers today to try to get an idea of what it may be. Two of them said they had never heard of such a thing and the third said it could be numerous things, from needing a reset to a lose wire.

 

Diagnosing weird things can be a pain in the butt. Trying to eliminate the list of possibilities may not be easy.

 

I hope the tranny fluid was changed. It should have been changed a while ago.