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Discussion Starter #1
I have a 2010 Mazda 3 2.0 Automatic Trans with 150K miles on it. I am getting the P0791 code and the AT light on dashboard comes on at around 40 miles per hour. The AT light comes on and off frequently but the CEL remains lit. Based on some readings I have been doing on the P0791, I have done the following:

  • Replaced the speed sensor (one that sits on top of the transmission)
  • Dropped the transmission pan, replaced the transmission filter, and replaced fluid with the proper fluid
The speed sensor was done about a year ago, but did the transmission fluid/filter this morning. After getting it all buttoned up, cleared the codes, took it for a ride and got the same P0791 code. I am out of ideas and was hoping for some other suggestions.
 

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Discussion Starter #3
The sensor that P0791 is associated with is the intermediate sensor, not the input sensor.
Are you saying there is a different sensor than the one that is accessible on top of the transmission? Is there a more official name which would allow me to search for it?
 

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First off, a disclaimer that I've never had to mess around with or even look for that sensor, so I have no hands-on experience with it. That said, the FSM I have shows 3 sensors on top of the transmission case, which are called: Input Speed Sensor, VSS, and Intermediate Sensor, located front to back in that order. And the manual also says that a P0791 relates to the Intermediate Sensor. However, it's strange that nothing turns up for anything called Intermediate Sensor in the online parts sites, so perhaps it has another name in the parts catalog. It's too dark now, but tomorrow I'll find out what's actually there on my 2011, and perhaps I can find part numbers which might help figure this out.
 

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Discussion Starter #5
First off, a disclaimer that I've never had to mess around with or even look for that sensor, so I have no hands-on experience with it. That said, the FSM I have shows 3 sensors on top of the transmission case, which are called: Input Speed Sensor, VSS, and Intermediate Sensor, located front to back in that order. And the manual also says that a P0791 relates to the Intermediate Sensor. However, it's strange that nothing turns up for anything called Intermediate Sensor in the online parts sites, so perhaps it has another name in the parts catalog. It's too dark now, but tomorrow I'll find out what's actually there on my 2011, and perhaps I can find part numbers which might help figure this out.
That would be great. Appreciate you taking the time to look.
 

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Discussion Starter #6 (Edited)
Looking at the Mazda Parts catalog, I do see these two:

Vehicle Speed Sensor - Mazda (FN01-21-550)

Vehicle Speed Sensor - Mazda (FN12-21-551A)

I have also read the problem could be the wiring or wiring harness.

According to my Amazon purchase history, the part below is what I purchased last year in trying to diagnose this issue. Pretty sure this is what I installed on the top of the transmission just under the battery box.

AUTEX Vehicle Speed Sensor VSS FN1221551A Compatible with Mazda 3 04-10/Mazda 6 04-07 09/Mazda CX-7 10/Mazda MX-5 08/Mazda Protege5 00-03/Mazda Protege5 02-03 (for Automatic Transmission)
 

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I was going to say you'll want to ensure the problem is not a wiring harness issue (chaffed or broken wire). Have you searched YouTube? A couple of years ago, I searched YouTube for Mazda 3 automatic transmission repairs and found some that detailed certain issues that would throw transmission codes. One video dealt with the pump.

Just throwing some things out there so you don't go throwing parts at it.
 

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Discussion Starter #8
I was going to say you'll want to ensure the problem is not a wiring harness issue (chaffed or broken wire). Have you searched YouTube? A couple of years ago, I searched YouTube for Mazda 3 automatic transmission repairs and found some that detailed certain issues that would throw transmission codes. One video dealt with the pump.

Just throwing some things out there so you don't go throwing parts at it.
I see a lot of related videos that addresses the transmission control module (TCM), but would like to rule out the speed sensors(s) first.
 

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I see a lot of related videos that addresses the transmission control module (TCM), but would like to rule out the speed sensors(s) first.
I forgot about the TCM being a possibility. If your car still has the original TCM, then that would be a miracle. I can't remember which version mine was upgraded to when it went out.
 

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Well, although it's difficult to believe, Mazda shows 3 speed sensors in their FSM, but there are only 2 (on my 2011 and also in the parts catalogs). So that also causes their documentation on the P0791 code to be unreliable. They show the mystery 'intermediate' speed sensor as the one closest to the firewall, on top of the tranny case. And I believe that's the one you have already replaced.

So at this point I'd suggest testing with a meter at both speed sensor connectors, before having to resort to replacing the TCM. The wire at 'A' is power (with key at on), and 'C' is ground. 'B' on the sensor connector is the signal wire and should have continuity to pin 'AC' on the TCM connector. PITA removing the battery stuff, but you would need to do that anyway in order to replace the TCM.
 

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Yes, removal of the battery box is a bit of a PITA. I've ripped apart the engine bay of my MZ3 so many times that it's a non-issue these days.

I am eager to see how this turns out for you, so please keep reporting your findings.
 

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Discussion Starter #12
So I replaced the input shaft sensor (FN1221551A) last year to no result. I am going to now replace the output shaft sensor (FN0121550) tomorrow to see if that fixes the problem. I am going to assume since it's output, I will be looking for that on the transmission further towards the driver wheel side? Haven't seen a diagram that lists the output shaft sensor location.

If that still doesn't so the trick, then I will focus on the TCM.
 

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Discussion Starter #13

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Discussion Starter #14
Pulled the batter box and took a picture. Screwdriver is pointing to what I believe is the input speed sensor that was replaced a while back. Not sure I see where the output speed sensor is located yet.

105459
 

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Discussion Starter #15 (Edited)
So looks like the sensor is the one that is pictured above towards the bottom. I noticed before taking it off that the wires were taped around the plug portion. Not sure that is factory or that means there is a wire connection issue. Anyway, here is a pic (that matches what I see on Amazon for FN01-21-550) removed.

105460
 

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I THINK the sensor you removed is the input speed sensor, but cannot say that for certain because of the Mazda FSM documentation issue that I mentioned previously. But there should be a part number on the factory sensor, which you can use for confirmation. I believe that 'tape' is actually the way it was wrapped by the factory.

Again, my choice would be to first test the wiring/connectors, particularly the (presumed) output speed sensor that you replaced previously. It would be a REALLY bad outcome if you replaced the sensor, then the TCM, only to find out that the actual problem is a simple wire/connector issue. All you would need to do is reconnect the PCM and jumper wire the battery to the cables. It only needs key on to check for power and ground, and the signal wire is just a resistance test. The full testing would include scope testing the signal waveform, but that's obviously not going to happen.
 

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Discussion Starter #17
I called two different Mazda dealers and got two different answers. One said FN01-21-550 was input sensor and the other said it was output sensor. Got the same mixed response for FN12-21-551A. Clearly both sensors are different and have different plugs.

The one I removed this morning has a 2 prong plug which matches what is referred to as the output shaft sensor in description and picture.

The one I replaced a year ago had the 3 prong plug which matches what is referred to as the input shaft sensor in description and picture.


Vehicle Speed Sensor - Mazda (FN01-21-550)
Transmission Output Shaft Speed Sensor.

105461



Vehicle Speed Sensor - Mazda (FN12-21-551A)
Transmission Input Shaft Speed Sensor.
105462
 

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Check out the parts diagrams on the Priority Mazda website. They're located in Tyson's Corner, Virginia. I've purchased many OE parts from them. The prices are great as is their service.
 

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I called two different Mazda dealers and got two different answers. ….
Well, their confusion is REALLY easy for me to believe and understand. The Mazda FSM says there are 3 speed sensors when there are clearly only 2 on the vehicle. This doc even includes a full diagnostic tree for all 3 sensors, including the one that doesn't exist. And it also says the 2 pin speed sensor is the input sensor and the 3 pin is the output sensor, however all the parts sites are saying it's the other way around. So it's totally obvious why everyone is confused by this mess. (I certainly am totally bewildered by it)

In looking at the specs for the 2 pin sensor (the one sure way to tell them apart), the FSM gives a test resistance spec for the sensor, if you're interested in testing the new and old sensors. Beyond that, I don't think it makes sense for me to give you any other advice based on what the Mazda FSM says, because it's already been proven to be completely out to lunch with these speed sensors. I no longer have any confidence that their diagnostic tree for the P0791 code is meaningful and reliable, and who knows at this point what that code actually is supposed to be indicating.

Good luck with the part replacement, and it would likely be a great help to others here if you post your eventual outcome.
 

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Discussion Starter #20
The part I ordered arrives on Wednesday at which time I will install, test, and come back here with results. And I do think Mazda needs to get their information straightened out.
 
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