Many Mazda 3 owners have found the variable AT warning light successfully solving the problem. And even dealers are often not that helpful in diagnosing the problem.
Also, the transmission will likely get stuck in third once the AT comes on. Still, if you stop the car and restart it, you’ll probably be able to drive normally afterwards, but that doesn’t solve the underlying problem.
The AT light on a Mazda 3 indicates a problem with the automatic transmission, the most common causes being a low transmission fluid level, a faulty TCM (Transmission Control Module), a TSS (Turbine Speed Sensor ) defective, faulty transmission and transmission solenoids.
Mazda 3 AT Light Meaning
The AT light, short for Automatic Transmission, is essentially the transmission’s check engine light. That means it can account for any and all potential issues with the transmission. Therefore, without a proper scan tool or professional diagnosis, it is almost impossible to tell what exactly is behind the warning light. In addition, since there are various causes, the transmission will automatically enter limp mode. That’s why it gets stuck in third.
So to begin with, you should get an OBD2 scan tool and start scanning for trouble codes. After that, if you get any of the following trouble codes, you’re in luck. However, if there are multiple trouble codes, it is likely that the transmission has failed or the TCM is faulty. Also, you may not get the corresponding P0700 trouble code with a faulty 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 – Transmission Fluid Pressure Low
Light Mazda 3 AT: causes and solutions
transmission fluid low
Although the transmission is a closed system, it is not immune to fluid leaks. That’s the first thing to start looking for, especially if you’re getting the P0868 trouble code. It’s also worth mentioning that old fluid can also be a problem, as it will lead to low pressure.
When it comes to the Mazda 3, the ideal is to replace your transmission fluid every 90,000 miles. So whatever the case, leaks or old fluid, replace the transmission fluid and fix the leaks.
Unfortunately, there is no easy way to replace or even top up your transmission fluid, so it is best to take the car to a repair shop. There, a top-up will probably cost you around €50 to €100, while a fluid shift will be closer to €200. It’s also important to mention that this is very time-sensitive work. Low transmission fluid can quickly cause irreparable damage to the transmission, so get the problem resolved as soon as possible.
Transmission control module failure
Although the TCM or any other control module rarely fails, it is a common problem on the Mazda 3. And since the TCM is responsible for reporting trouble codes, diagnosing it can be a nightmare because it can throw up various false trouble codes at the time. Still, Mazda dealers are very aware of these issues and will likely help you if you get stuck trying to diagnose it.
The reason these TCM units fail is that they are mounted on the transmission itself, causing the TCM to overheat and fail prematurely. So what a lot of people do after a replacement is relocate the TCM to the firewall. You can easily do this with some self-tapping screws.
As for replacement options, you can repair your original TCM, 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 usually costs around €330, while a new one costs between €500 and €700. As for labor costs, it will probably be around €100, but it can also do it yourself quite easily. But before we explain how, here’s a list of other symptoms you may notice with a bad TCM.
TCM symptoms:
Hard shift to drive and reverse
Stuck in third gear (limp mode)
Trouble codes P0700, P0753, P0758, P0763
How to replace the TCM:
Disconnect the battery and remove it from the engine compartment.
Remove the plastic battery tray by removing the two bolts holding it down.
Disconnect the ECU connectors (mounted on the side of the battery tray) and remove the battery tray
You will now find the TCM under the metal battery holder where the tray was mounted.
The TCM is held on by three bolts, so remove them, disconnect the TCM and replace with a new one.
And finally, it’s a good idea to relocate the TCM to the firewall to avoid the same problems in the future.
Here’s 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 RPM sensor to determine when to shift. It’s also partly responsible for cruise control, so you may be in trouble if the TSS fails. Other than that, you may notice irregularities in the RPM gauge, the vehicle refusing to shift, and making hard shifts.
Unfortunately, while the TSS is hardly any different from speed sensor or even cam/crank position sensors, it is significantly more expensive. TSS for a Mazda 3 is between €100 and €150, and if you do it through a dealer it will be closer to €200.
But on a positive note, diagnosing and replacing it is pretty easy. The fault code you’re looking for is P0715, and the labor cost probably won’t exceed $100. Plus, you can replace it at home with a few basic tools; here is how.
How to replace the turbine speed sensor:
Disconnect and remove the battery. Disconnect the negative terminal first.
Unbolt the plastic battery tray and disconnect the ECU sitting to the side. Then remove the plastic tray.
Now under the metal shelf where the tray was, locate the TSS on the transmission. To find the TSS, you can use a flashlight or take a video with your phone.
Once you locate it, disconnect the TSS and remove the single 10mm bolt holding it down.
Lastly, twist and turn the sensor while pulling to remove it.
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 need to address when talking about solenoids is to check the TCM symptoms and trouble codes first. This is because a faulty TCM will throw solenoid trouble codes, and solenoids rarely fail. Also, it’s unlikely that all three solenoids will fail at once, but it’s best to replace all three while you’re at it.
Unfortunately, replacing solenoids is expensive. A single solenoid is around $50 at AutoZone, and an OEM unit is just under $100. The cost of labor is also high and not a do-it-yourself job. With the labor costs and the three solenoids, the replacement cost is probably between €300 and €450. But again, check the TCM first, and here are the rest of the solenoid symptoms.
Transmission Solenoid Symptoms:
slow change
I can’t change neutral
can’t downshift
Stuck in a certain gear
abrupt change
sliding gears
Erratic switch points
lazy 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 occur. But make sure you do a full diagnosis before doing it because it is an expensive job.
A new transmission is probably around €4,000, while a rebuilt is half that, at just under €2,000. However, you can try your luck getting a used one from a wrecked vehicle, and that will cost you around $500 to $900.
However, the cost of labor is inevitably high and you will likely end up paying around 15 hours of labor. And with an average repair shop hourly rate of €70, the total labor cost comes to just over €1,000.
What type of transmission does Mazda 3 use?
The Mazda 3 can have a manual, automatic or CVT transmission. Automatic transmissions are available with either four or five gears, both of which are developed in conjunction with Ford. The two transmissions are the FN4A-EL and the FS5A-EL.
Are Mazda automatic systems reliable?
Yes, Mazda automatic transmissions are exceptionally reliable. Between the years 2005 and 2020, there were no major issues other than 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 made by Ford for its own models. However, Aisin makes Mazda’s CVT transmissions, and Jatco used to make some automatics for Mazda up until the 1990s.
How long do Mazda 3 automatic transmissions last?
According to https://www.transmissionrepaircostguide.com/, Mazda 3 transmissions typically last between 130,000 and 180,000 miles. However, transmission life depends somewhat on the quality of maintenance and your driving style.
Conclusion
At the end of the day, the most likely cause of an AT light on a Mazda 3 is a faulty TCM. Mazda TCMs are a common point of failure and will also display a number of different symptoms and several different trouble codes. So, I would definitely start with that. Still, if you get a more accurate fault code, you can tackle that problem yourself and see if it resolves the issue.
Some of them could be the turbine speed sensor, transmission fluid leaks, bad solenoids, and transmission failure. Of those, the turbine speed sensor and transmission fluid have definite trouble codes. And while the solenoids do as well, those trouble codes are likely to show up if the TCM is misfiring, so again, a thorough inspection is required.
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