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Discussion Starter · #1 ·
Hello.

My son has a 2010 Mazda 3i with 99K miles. The AT warning light comes on intermittently. Sometimes when it comes on, the car doesn't want to shift into 4th gear. For starters, the transmission fluid was drained and replaced, but that didn't seem to help. Once when the AT light was on and when it wasn't shifting into 4th, we turned off the engine, and when we restarted, the light went out and the car seemed to drive normally.

The OBD2 error codes are:
P0734 - Gear 4 - incorrect ratio
P0766 - Shift solenoid D - performance or stuck off
P0841 - Transmission fluid pressure (TFP) sensor A - range/performance problem
U0402(92) TCM - Invalid data received - transmission control module

Does anyone know what is the problem? Is the TCM going bad? Is the shift solenoid D going bad? Is the TFP sensor A going bad? All of the above?

Thanks for your help,
Tim
 

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The TCM is probably going bad. There's a guy on one of the Facebook groups that can rebuild it for about $300, or about a third of the cost of a new one. But because I haven't been on that hive of scum and villainy in nearly a year, I can't remember his name. But I promise, the guy does exist, with a whole bunch of people having great results.
 

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Discussion Starter · #3 ·
The TCM is probably going bad. There's a guy on one of the Facebook groups that can rebuild it for about $300, or about a third of the cost of a new one. But because I haven't been on that hive of scum and villainy in nearly a year, I can't remember his name. But I promise, the guy does exist, with a whole bunch of people having great results.
Thank you for your reply. I found this one on Facebook: https://shop.modulerepairpro.com/
They charge $249 to repair with a lifetime warranty.
 

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Outstanding!

When you mount the new one, move it onto the firewall. How I did my MZ5 was, I took some rubber fuel line, split it, and wrapped it around the brake lines coming down from the master cylinder. I then zip tied the TCM there.

Whoever the genius was that though mounting a computer to a hot assed transmission needs to be shot!

Also, while you're in there, I highly suggest you change the transmission mount. They often fail, and you've already done 95% of the work getting to it.
 

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Hello.

My son has a 2010 Mazda 3i with 99K miles. The AT warning light comes on intermittently. Sometimes when it comes on, the car doesn't want to shift into 4th gear. For starters, the transmission fluid was drained and replaced, but that didn't seem to help. Once when the AT light was on and when it wasn't shifting into 4th, we turned off the engine, and when we restarted, the light went out and the car seemed to drive normally.

The OBD2 error codes are:
P0734 - Gear 4 - incorrect ratio
P0766 - Shift solenoid D - performance or stuck off
P0841 - Transmission fluid pressure (TFP) sensor A - range/performance problem
U0402(92) TCM - Invalid data received - transmission control module

Does anyone know what is the problem? Is the TCM going bad? Is the shift solenoid D going bad? Is the TFP sensor A going bad? All of the above?

Thanks for your help,
Tim
Hello.

My son has a 2010 Mazda 3i with 99K miles. The AT warning light comes on intermittently. Sometimes when it comes on, the car doesn't want to shift into 4th gear. For starters, the transmission fluid was drained and replaced, but that didn't seem to help. Once when the AT light was on and when it wasn't shifting into 4th, we turned off the engine, and when we restarted, the light went out and the car seemed to drive normally.

The OBD2 error codes are:
P0734 - Gear 4 - incorrect ratio
P0766 - Shift solenoid D - performance or stuck off
P0841 - Transmission fluid pressure (TFP) sensor A - range/performance problem
U0402(92) TCM - Invalid data received - transmission control module

Does anyone know what is the problem? Is the TCM going bad? Is the shift solenoid D going bad? Is the TFP sensor A going bad? All of the above?

Thanks for your help,
Tim
Highly possible your TCM is bad.
My 2009 started thumping thru gears, AT light came on saying input speed sensor fault. Swapped sensor with output ( they are the same) fault code didn’t change.
Replaced TCM (expensive) issue fixed.
Was giving phantom fault codes.
Mazda dealer said they are replacing many of these TCM’s as 3’s get older.
 

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Hi, I recently had similar issue with my 2007. At 1st the engine light came on as well as the trac light but both went away for like a week and then came back. During all this time the car drove normal. The codes I got were all U codes with a likely fix suggestion of “replace battery”. I had the battery tested and it was good. Another fix suggested was to test alternator and that was good too. These are all “likely fixes” based on the bluedriver obd2 scan reading, it gives you a list of possible fixes. Everything pointed to a bad tcm so I took it to the dealer to have it checked because I wanted to make sure it wasn’t something else that I was not catching. They did a reset and had it relearn (the tcm i presume) and told me the car module had lost communication which caused the car to generate the check engine light and produce codes but that it should be good now. Well, the car drove fine for like a day before the engine light coming back but this time with the AT light as well. This time I could feel some resistance from car when the AT light would randomly flash. I took it back to the dealer and they checked it again and further diagnosed it. They told me the tcm was faulty and needed replacement. They said it would be covered for free under the “P warranty” and that they recommended I change the trans mount because it was worn out and would help protect the tcm further from bouncing/hitting? I asked if they could place the module elsewhere to avoid the heat getting to it (because i read that causes tcm to fail) and they said they could not because it had to be placed where it belongs. Since they weren’t charging me To replace it i wasnt going to argue with them because i know replacing them is not cheap. I also didnt question this p warranty thing because i was scared they would then say i did not have the warranty afterall. My car is 13 yrs old and I have maintained it well since I have had it, Anyways, i just got the car back yesterday and it drives fine with no lights coming on. I tried looking up what p warranty is but the only thing i could find is that some mazdas have a lifetime warranty for transmission and powertrain issues, this includes replacing the tcm. Some googling led me to if you are the original owner of the car and bought it from an authorized dealer, this is covered. I am the original owner so I don’t know if that is why it was free to replace?However, I took it to a different mazda dealer for repair than the one I had bought car from. Unsure if this is in general with all new bought mazda’s or certain ones. Maybe if you are the original owner try to find out and perhaps they can cover it free of charge? The only thing I paid for was to replace trans mount which was $409 and I know I could have paid way less but they had the car already and I figured it was them replacing it and they give you 12 months/12K miles warranty if the mount goes bad plus I needed my car back asap to commute to work. Best of luck and greetings from California!
 

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Discussion Starter · #7 ·
Outstanding!

When you mount the new one, move it onto the firewall. How I did my MZ5 was, I took some rubber fuel line, split it, and wrapped it around the brake lines coming down from the master cylinder. I then zip tied the TCM there.

Whoever the genius was that though mounting a computer to a hot assed transmission needs to be shot!

Also, while you're in there, I highly suggest you change the transmission mount. They often fail, and you've already done 95% of the work getting to it.
I asked the repair shop for status on my TCM. This was their response:

"BE ADVISED, THAT FOLLOWING CODES FROM YOUR LIST ARE NOT COMING FROM THE TCM ITSELF, BUT A TRANSMISSION INTERNAL ISSUE/S:
OBDII: Gear 4 - incorrect ratio
OBDII: Transmission fluid pressure (TFP) sensor A - range/performance problem"

I'm still wondering if the TCM was causing all the faults. I'm debating whether I should replace the transmission pressure switch before I button things up. Aftermarket switches seem inexpensive.

Thank you for the suggestion to replace the transmission mount. Any recommendations on an aftermarket switch or would you stick with stock?

I should receive my TCM today.
 

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Discussion Starter · #8 ·
Hi, I recently had similar issue with my 2007. At 1st the engine light came on as well as the trac light but both went away for like a week and then came back. During all this time the car drove normal. The codes I got were all U codes with a likely fix suggestion of “replace battery”. I had the battery tested and it was good. Another fix suggested was to test alternator and that was good too. These are all “likely fixes” based on the bluedriver obd2 scan reading, it gives you a list of possible fixes. Everything pointed to a bad tcm so I took it to the dealer to have it checked because I wanted to make sure it wasn’t something else that I was not catching. They did a reset and had it relearn (the tcm i presume) and told me the car module had lost communication which caused the car to generate the check engine light and produce codes but that it should be good now. Well, the car drove fine for like a day before the engine light coming back but this time with the AT light as well. This time I could feel some resistance from car when the AT light would randomly flash. I took it back to the dealer and they checked it again and further diagnosed it. They told me the tcm was faulty and needed replacement. They said it would be covered for free under the “P warranty” and that they recommended I change the trans mount because it was worn out and would help protect the tcm further from bouncing/hitting? I asked if they could place the module elsewhere to avoid the heat getting to it (because i read that causes tcm to fail) and they said they could not because it had to be placed where it belongs. Since they weren’t charging me To replace it i wasnt going to argue with them because i know replacing them is not cheap. I also didnt question this p warranty thing because i was scared they would then say i did not have the warranty afterall. My car is 13 yrs old and I have maintained it well since I have had it, Anyways, i just got the car back yesterday and it drives fine with no lights coming on. I tried looking up what p warranty is but the only thing i could find is that some mazdas have a lifetime warranty for transmission and powertrain issues, this includes replacing the tcm. Some googling led me to if you are the original owner of the car and bought it from an authorized dealer, this is covered. I am the original owner so I don’t know if that is why it was free to replace?However, I took it to a different mazda dealer for repair than the one I had bought car from. Unsure if this is in general with all new bought mazda’s or certain ones. Maybe if you are the original owner try to find out and perhaps they can cover it free of charge? The only thing I paid for was to replace trans mount which was $409 and I know I could have paid way less but they had the car already and I figured it was them replacing it and they give you 12 months/12K miles warranty if the mount goes bad plus I needed my car back asap to commute to work. Best of luck and greetings from California!
That's great that they replaced your TCM for free under the P warranty. Unfortunately, we are not the original owners. Thank you for sharing the info.
 

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Discussion Starter · #10 ·
Never buy aftermarket sensors. They never work correctly. Buy once, cry once.
Okay, noted: only use OEM sensors.

I received the repaired TCM and decided to install it without making any additional repairs to see if the codes clear.

The good news is that all the old codes are gone. The bad news is that there's a new code:

P0715
Raw code: 0715
ECU: Transmission control unit
Status: Permanent
OBDII: Turbine shaft speed (TSS) sensor - circuit malfunction

I tried to clear the code via my OBD2 device with no success. I took it on a short test drive and the car seems to drive okay.

I'm thinking that the TCM still has issues. I feel like I'm in an insidious Wack-a-mole game. Any thoughts?
 

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Found this basic plan of attack if you hadn't already seen it.

 
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