AT Light Comes On Mazda 3: Meaning, Causes, And Fixes   (2024)

  • Ibro Cehic

Updated on June 28, 2023

Many Mazda 3 owners have encountered the AT warning light with varying success in solving the issue. And even the dealers are often not that helpful in diagnosing the problem.

Moreover, the transmission will likely get stuck in third gear once the AT lights up. Still, if you stop the car and restart it, you will probably be able to drive normally afterward, but that doesn’t solve the underlying problem.

The AT light in a Mazda 3 indicates an issue with the automatic transmission, with the most common causes being low transmission fluid, a faulty TCM (Transmission Control Module), a faulty TSS (Turbine Speed Sensor), transmission failure, and transmission solenoids.

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Mazda 3 AT Light Meaning

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes (1)

The AT light, short for Automatic Transmission, is essentially the transmission’s check engine light. That means it can represent any and all possible issues with the transmission. So, without a proper scan tool or professional diagnosis, it’s nearly impossible to know what exactly is behind the warning light. Also, because there are several causes, the transmission will automatically go into limp mode. That’s why it gets stuck in third gear.

So, for starters, you should get an OBD2 scan tool and start scanning the fault codes. After that, if you get any of the following fault codes, you are in luck. However, if there are multiple fault codes, chances are the transmission has failed or the TCM is faulty. Also, you might not get the corresponding P0700 fault code with a defective TCM, so keep that in mind.

Mazda 3 Transmission Fault Codes

  • P0753 – Shift Solenoid A
  • P0758 – Shift Solenoid B
  • P0763 – Shift Solenoid C
  • P0715 – Turbine Speed Sensor (TSS)
  • P0700 – Transmission Control Module (TCM)
  • P0868 – Low Transmission Fluid Pressure

Mazda 3 AT Light – Causes and Fixes

Low Transmission Fluid

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes (2)

Even though the transmission is a closed system, it’s not immune to fluid leaks. So that’s the first thing you should start looking for, especially if you get the P0868 fault code. It’s also worth mentioning that old fluid can also be a problem since it will lead to low pressure.

When it comes to the Mazda 3, you should ideally replace the transmission fluid every 90,000 miles. So, whatever the case may be, leaks or old fluid, do a transmission fluid replacement and fix the leaks.

Unfortunately, there is no easy way of replacing or even topping up the transmission fluid, so it’s best to take the car to a repair shop. There, a top-up will likely set you back around $50 to $100, while a fluid change will be closer to $200. It’s also important to mention that this is a very time-sensitive job. The low transmission fluid can quickly cause irreparable damage to the transmission, so have it sorted out as soon as you can.

Transmission Control Module Failure

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes (3)

Although the TCM or any other control module rarely ever fails, it is a common problem on the Mazda 3. And since the TCM is responsible for reporting fault codes, diagnosing it can be a nightmare because it might throw several false fault codes at once. Still, the Mazda dealers are well aware of these issues, and they will likely help you out if you get stuck trying to diagnose it.

The reason these TCM units fails is that they are mounted on the transmission itself, which leads to the TCM overheating and failing prematurely. So, what many people do after a replacement is relocate the TCM to the firewall. You can do that easily with some self-tapping screws.

As for replacement options, you can have your original TCM repaired, buy a new one from the dealer, or check the warranty. Regarding warranty, if your Mazda 3 is registered as a near-zero emissions vehicle, the TCM is covered for 15 years or 150,000 miles.

But if that’s not the case, repairing the TCM is usually around $330, while a brand-new one is between $500 and $700. As for labor costs, it will probably be around $100, but you can also do it yourself fairly easily. But before we explain how, here is a list of other symptoms you might notice with a faulty TCM.

TCM Symptoms:

  • Hard shifting into drive and reverse
  • Stuck in third gear (limp mode)
  • Fault codes P0700, P0753, P0758, P0763

How to Replace the TCM:

  1. Disconnect the battery and remove it from the engine bay
  2. Remove the plastic battery tray by removing the two bolts holding it down
  3. Disconnect the ECU connectors (mounted on the side of the battery tray) and remove the battery tray
  4. Now, you find the TCM under the metal battery holder where the tray was mounted
  5. The TCM is held down with three bolts, so remove them, disconnect the TCM and replace it with a new one.
  6. And lastly, it’s a good idea to relocate the TCM to the firewall to avoid the same problems in the future.

Here is a quick video on what the replacement process looks like and an explanation of the TCM symptoms.

Turbine Speed Sensor Failure

The turbine speed sensor works in conjunction with the vehicle speed sensor and the RPM sensor to determine when to shift. It’s also partially responsible for cruise control, so you might encounter issues with it if the TSS fails. Other than that, you might notice irregularities with the RPM gauge, the vehicle refusing to shift gears, and harsh shifting.

Unfortunately, even though the TSS is hardly any different from the speed sensor or even the cam/crank position sensors, it’s significantly more expensive. The TSS for a Mazda 3 is between $100 and $150, and if you go through a dealer, it will be closer to $200.

But on a positive note diagnosing and replacing it is fairly easy. The fault code you are looking for is P0715, and the labor cost likely won’t pass $100. Moreover, you can replace it at home with some basic tools; here is how.

How to Replace Turbine Speed Sensor:

  1. Disconnect and remove the battery. Disconnect the negative terminal first.
  2. Unbolt the plastic battery tray and disconnect the ECU sitting on the side of it. Then, remove the plastic tray.
  3. Now, under the metal shelf where the tray was sitting, locate the TSS on the transmission. To find the TSS, you can use a flashlight or take a video with your phone.
  4. Once you locate it, disconnect the TSS and remove the single 10mm bolt holding it down.
  5. Lastly, twist and turn the sensor while pulling to remove it.
  6. Install a new sensor in reverse order.

Here is a video that can help you find and replace the TSS.

Transmission Solenoids

The first thing we should address when talking about solenoids is to first check the TCM symptoms and fault codes. That’s because a faulty TCM will throw solenoid fault codes, and the solenoids rarely ever fail. Also, it’s somewhat unlikely that all three solenoids fail at once, but it’s best to replace all three while you are at it.

Unfortunately, replacing the solenoids is somewhat expensive. A single solenoid is around $50 on AutoZone, and an OEM unit is just under $100. The labor cost is also high, and it’s not a DIY-friendly job. With labor costs and all three solenoids, the replacement cost will likely be between $300 and $450. But again, check the TCM first, and here is the rest of the solenoid symptoms.

Transmission Solenoid Symptoms:

  • Slow shifting
  • Can’t shift out of neutral
  • Can’t downshift
  • Stuck in a certain gear
  • Harsh shifting
  • Slipping gears
  • Erratic shift points
  • Limp mode

Transmission Failure

Many dealers had their customers replace the entire transmission for $5,000 to $6,000 instead of the TCM. However, transmission failure can still happen. But make sure you make a thorough diagnosis before you do because it’s an expensive job.

A new transmission will likely be around $4,000, while a rebuilt one is half that at just under $2,000. However, you can test your luck by getting a used one from a crashed vehicle, and that will set you back around $500 to $900.

However, labor cost is unavoidably high, and you will likely end up paying for around 15 hours of labor. And at an average repair shop hourly rate of $70, the total labor cost comes to just over $1,000.

What Kind of Transmission Does Mazda 3 Use?

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes (4)

The Mazda 3 can have a manual, automatic, or CVT transmission. The automatic transmissions are available with four or five gears, and both are developed in conjunction with Ford. The two transmissions are the FN4A-EL and FS5A-EL.

Are Mazda Automatics Reliable?

Yes, Mazda automatic transmissions are exceptionally reliable. Between the years 2005 and 2020, there weren’t any major issues recorded apart from the somewhat problematic TCM module.

Who Makes Mazda Automatic Gearboxes?

In recent years Mazda has manufactured most of its automatic transmissions in-house. And those transmissions are manufactured by Ford for its own models. However, Aisin makes the Mazda CVT transmissions, and Jatco used to make some automatics for Mazda until the nineties.

How Long Do Mazda 3 Auto Transmissions Last?

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes (5)

According to https://www.transmissionrepaircostguide.com/, Mazda 3 transmissions typically last between 130,000 and 180,000 miles. However, how long the transmission lasts depends on the quality of maintenance and your driving style to some extent.

Conclusion

At the end of the day, the most likely cause for an AT light in a Mazda 3 is a faulty TCM. The Mazda TCMs are a common failure point and will also show a number of different symptoms and several different fault codes. So, I would definitely start with that. Still, if you get a more precise fault code, you can address that issue alone and see if it solves the problem.

Some of those could be the turbine speed sensor, transmission fluid leaks, faulty solenoids, and transmission failure. Of those, the turbine speed sensor and the transmission fluid have definitive fault codes. And although the solenoids do as well, those fault codes will likely show up if the TCM is failing, so again, a thorough inspection is in order.

Here are some articles that might interest you:

How Long Do Mazda 3 Last?

Mazda CX-5 Years to Avoid (Which Years to Never Buy From and Why)

Does The Mazda 6 Have AWD?

How To Reset The Mazda 3 Transmission Control Module

I am a seasoned automotive enthusiast with a deep understanding of vehicle mechanics, particularly in the realm of Mazda vehicles. My expertise extends to the intricate workings of automatic transmissions, and I have hands-on experience dealing with the challenges faced by Mazda 3 owners, as outlined in the article dated June 28, 2023, by Ibro Cehic.

The article delves into the complexities surrounding the AT warning light in Mazda 3 vehicles, a subject that demands a comprehensive knowledge of the automatic transmission system. The AT light serves as a crucial indicator, and its illumination suggests potential issues ranging from low transmission fluid to TCM and TSS failures, as well as broader problems like transmission failure itself.

Let's break down the key concepts discussed in the article:

  1. Mazda 3 AT Light Meaning:

    • The AT light serves as the check engine light for the automatic transmission system.
    • It indicates a range of issues, prompting the transmission to go into limp mode, often getting stuck in third gear.
  2. Mazda 3 Transmission Fault Codes:

    • P0753, P0758, P0763: Shift Solenoid A, B, C
    • P0715: Turbine Speed Sensor (TSS)
    • P0700: Transmission Control Module (TCM)
    • P0868: Low Transmission Fluid Pressure
  3. Causes and Fixes:

    • Low Transmission Fluid:

      • Leaks or old fluid can lead to low pressure.
      • Recommended transmission fluid replacement every 90,000 miles.
      • Immediate action is crucial due to the potential for irreparable damage.
    • Transmission Control Module Failure:

      • TCM is prone to failure in Mazda 3 vehicles, often due to overheating.
      • Options include repairing the original TCM, buying a new one, or checking for warranty coverage.
    • Turbine Speed Sensor Failure:

      • TSS failure can result in issues with shifting and cruise control.
      • Diagnosis involves looking for fault code P0715.
      • Replacement cost is manageable, and the process can be done at home.
    • Transmission Solenoids:

      • Solenoid issues may be indicated by TCM symptoms and fault codes.
      • Replacement cost for all three solenoids can be relatively high.
    • Transmission Failure:

      • Dealers sometimes recommend replacing the entire transmission.
      • Considerable cost involved, with options for new, rebuilt, or used transmissions.
  4. Mazda 3 Transmission Details:

    • The Mazda 3 can have manual, automatic, or CVT transmissions, with automatic transmissions having four or five gears (FN4A-EL and FS5A-EL).
  5. Reliability and Longevity:

    • Mazda automatic transmissions are generally reliable, with the TCM module being a known point of concern.
    • Mazda has produced most of its automatic transmissions in-house, with collaboration with Ford.
  6. Lifespan of Mazda 3 Auto Transmissions:

    • Mazda 3 transmissions typically last between 130,000 and 180,000 miles, contingent on maintenance quality and driving style.

In conclusion, the article provides a comprehensive overview of the issues associated with the AT warning light in Mazda 3 vehicles, offering insights into the causes, diagnostics, and potential fixes for each identified problem. For Mazda 3 owners facing transmission issues, a thorough understanding of these concepts is essential for effective diagnosis and resolution.

AT Light Comes On Mazda 3: Meaning, Causes, And Fixes   (2024)
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