cmcgregor Dork
9/18/19 5:36 p.m.

While I was traveling for work, the transmission on our '09 Mazda 5 started freaking out. It went into limp mode on my wife twice, and lit the AT and check engine lights. The code was U0101. If you clear the codes, it will drive fine for a while, then start trying to randomly downshift into 3rd while you're tooling down the road and flash the AT light.

I pulled the TCM and cleaned everything up, but that didn't seem to help. Fuses and grounds seem fine, this has always been a CA car so corrosion isn't an issue. To me this looks like a textbook TCM failure, but is there anything else I should check before I drop $400 on a new one?

John Welsh
John Welsh Mod Squad
9/18/19 6:21 p.m.

Check for used ones and drop $100-ish instead? 

Streetwiseguy MegaDork
9/18/19 6:30 p.m.

Module failures are exceedingly rare in any modern automobile.  If there is a remotely common issue, it is well documented in the intertubes. 

Power supplies good?  Grounds good?  No fretting on any terminals?  No other modules on the bus pulling it down? No wires rubbed through making intermittent contact with ground?  No sensors in the trans pulling the 5V ref to ground?

There are a lot of things to check before you pull the pin on a module, which you will likely have to have flashed to work at all. 

cmcgregor Dork
9/18/19 6:37 p.m.

The reason I'm jumping to that conclusion is that it's a fairly well-documented problem with these, which also makes me hesitant to buy a used one. Grounds and power supplies are good, nothing else is acting up, and so far as I can tell no shorts. This particular module is plug and play. 

Thanks for the suggestions, I will check more thoroughly but electrical troubleshooting is not my forte. 

cmcgregor Dork
9/27/19 5:52 p.m.

Update - replaced the module (~30 minutes, since I've had all this stuff out more than once now) and the problem is solved. I still kinda hate the van, but the transmission is behaving again. 

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