New guy with a question U0101

Hello All, I feel a introduction is needed, My name is Michael I have a wife and 4 kids (one being 19 but wont leave the nest) We just purchased a 2008 Mazda5 a year ago because our Kia Sedona was killing our pockets. This car has been great to us so far. Now I am somewhat capable of working on my own cars, just need to be pointed out when it comes to electrical aspect of a vehicle.

I hope someone can help me out here, I am almost close to classifying myself as a middle class individual but I'd be lying. I do work a ft and pt job but bringing this to a dealer is not an option for me unfortunately. This car is all I really have.

Now heres my issue, you may of heard it a dozen times, AT with a Check engine light! The fluid is fine, and this is only an occasional problem. Started as of yesterday I was driving down the road and the car jolted and I s*** my pants. The AT light came on as well as the CEL, the CEL stayed on but the other went away and the car shifted great. Now this morning I went for a ride, the AT light started on with the car. I noticed it was a bit sluggish getting around, until another hard shift. I pulled over and put her into manual mode just to watch. I was in 5th gear and felt the jolt, looked it popped into 4th the immediately back into 5th. I know some of you will tell me to google this as it is a common problem, and I have. Some of these topic tend to go off track, and I afraid this car wont allow me that type of time. This is really the only thing I have beside my family. So I apologize for having to put my faith into you kind readers.

Autozone came up with a U0101 Lost communication with tran Control Module, months prior there wasn't communication with the pcm? as well.

Heres a pic of my girl... we have a sense of humor. This is us at a car show others thought we weren't worthy of!

(rlaugh) welcome!

More history on the car if you have it. If you don’t’ (since it is used so anything is possible) than you are SOL outside of dealer diagnostics or work by the one code you have, which I’m sure you have googled to death. Keep in mind this seems to be a CANbus error so it applies to all Mazdas. Focus you googling for Mazda3 or mazda6 with the same error code and you’ll get better results.

How do you know the AT fluid is fine? Did you just change it? Have it tested? It probably doesn't hurt to do a complete change and see how it drives.

I haven't change the fluid in the transmission since I bought it. I am going strictly by color, I don't know if it's the right blend or brand. Now heres a silly question, I don't mind bringing this to a specialist, I just can't afford guesses. I read a lot of people being miss directed in this matter. Will there scanner be more conclusive than Autozone's broad U0101 code in your opinion?

To answer the other question, I only know of the history of it with me. I don't have a maintenance plan for it. It ran great this whole year, seems to me (and this could be coincidental) The few times this has happened was after a couple days of rain, and a pressure washing. The only other time I had a problem with it was trying to cross a highway median with it and scuffed the bumper the 2nd day I had it.

No one on the internet is going to be able to diagnose your car and your experience is not common in this forum. The following are some suggestion on some things you can do and pure SPECULATIONS based on the few bit of info provided.

The transmission is a unique monster in of itself. All shops are capable of simple computer diagnostic but most cannot rebuild one. You should take it to a transmission shop like Aamco or dealer if you are really concerned.

You said you also got a CEL. Did you pull a DTC for that?? Based on your description I would speculate a faulty TCM first. Other possibilities could be a rodent may have chewed some wires as this is a communication error, or PO messed with something electrical and screwed something up. There is no history on the car (flood car or PO splice wrong wires/bad connection).


Do some simple stuff first to eliminate and pray it is an odd occurrence.
1) Make sure you battery is in good condition (Autozone does free battery tests).
2) Disconnect battery (while Autozone is doing their free test) and hold down the brakes for 10 secs, turn the ignition to the “on” position, then take key out. This ensures discharging reserve energy in capacitors to reset everything.
3) Reconnect battery, start it up, and let it idle for ~10-15 mins so the car can go through the ideal lean procedure.
4) You need to have a diagnostic device handy to catch any CEL or AT light that might come up -or do this in Autozone’s parking lot in the event something poop up )
5) If you get the same AT code, check wires in/around the TCM unit (easier said than done) or you can simply try and swap in a replacement TCM. If you are willing to take a leap of faith, you can go ahead and replace the TCM with a used one (have a friend/neighbor with a Mz3 or Mz5 of the same gen?). Cross check if Mazda3 used the same part as they are cheap and avail on ebay for <$100. If codes still persists with a replacement TCM (non DOA unit), start looking at wires (tracing wires suck).


If it were me and I don’t get a CEL and only the same AT light comes up after the reset, I would take the leap of faith and get a used TCM (double check if Mz3 of the same gen uses the same part) and swap it in before taking it in to get diagnose.

Silentnoise, I appreciate your input. I know it's hard to diagnose over the internet. I more or less guess I am looking for a checklist all in one page like you've just done for me (thank you) The CEL code was U0101 lost communication with tran control module. The CEL has been on, I double checked it and came up with the same code. I left the battery un-hooked for a couple hours as someone told me it will allow the computer to rest itself when I hook it back up. I just went for a ride with her about 40 minutes or so and the AT only happened once briefly. In a parking lot at like 5 mph

It has been a while since the last reply. I am having the same issue.

I have the u0101 code and CEL. It happened over the weekend and I was able to drive it home. The car will drive really slow, when I put it on Reverse, the car would jerk. I bought it to a local Mechanic, they said that the ECM is cooked and needs replacement.

I am wondering where the ECM is located, is it the device to the left of the Battery with 2 bunches of wires plugged in?

How much does a job like this cost? Thanks for any help.

Sorry to bring up an old thread, but my wife's Mazda 5 is having the exact same problem. Car has 97,000 miles. She did purchase a 100,000 mile extended warranty.

1) Even if she did not have the extended warranty, if this is a known factory defect, wouldn't it be covered regardless if she had an extended warranty or not?

2) I am classified as stage 4 cancer, even though I am in remission, money is non-existent, and I had to put a rental car on my credit card, plus I know the dealer charges $$$ just to diagnosis the problem. Are any of my out of pocket costs covered under Mazda's warranty & extended warranty? (oh yeah, forgot to include towing charges)

3) I'll try unhooking the battery, and hope that works. Also, another member suggested there might be a tcm fuse - so I will check for that also.
Thank you in advance for any advice/help