Driver side door lock issue (2015 Mazda CX-5)

Hey everyone,

I recently purchased a 2015 Mazda CX-5 Touring with 78k miles for my daughter. Just about four weeks after the purchase, the driver's side door lock started acting strangely. The key fob wouldn't lock the driver's side door, even after replacing the battery and trying the spare key fob. Interestingly, using the key blade from the fob allowed me to lock and unlock the doors. However, both the key fob and the master switch on the driver's side failed to lock the driver's side door, although they worked fine for the other doors and hatch. Additionally, the car horn no longer sounded after locking the door twice with the key fob.

Despite trying various troubleshooting steps, including replacing the door actuator with a new part from Mazda, borrowing a friend's driver's side door master switch, and swapping out the fuse twice, the issue persisted. Online research led me to suspect the rear BCM (body control module) might be faulty, but testing disproved this theory.

In a last-ditch effort, I downloaded and installed Forscan scanning software and purchased a Forscan scanner OBDC reader from Amazon. Surprisingly, it worked, and I was able to retrieve error codes: B1172 and B126A. After researching these codes, I discovered that B1172 indicates a circuit short to ground in the rear BCM, while B126A relates to a symptom with the driver's door interior lock.

I apologize for the lengthy explanation, but I wanted to provide a detailed account of my troubleshooting efforts in the hopes of assisting others facing similar issues. If anyone has insight into what these codes mean before I resort to taking the CX-5 to a shop, I would greatly appreciate it.

Thank you in advance!

Jat
 
Curious if your rear wiper and cargo light still work? When my rear BCM went out those two items didn’t work in the rear.

Is your cargo area running the stock incandescent bulb or was it upgraded to an LED?
 
Hi, yes, the lights are still lid. No, it's still the original incandescent bulb as I was just getting something out of the trunk last night and said, boy this light is horrible compared to my newer model. Thanks!
 
Maybe this will help with troubleshooting?
 

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Hi,
Quick question. On the master switch on the driver side, it you remove it from the door and on the back side of that master switch, you have two (2) connectors, do you supposed to have power to the top connector? I have power to the bottom connector, but not for the top connector?

Thanks.
 
Hey everyone,

I recently purchased a 2015 Mazda CX-5 Touring with 78k miles for my daughter. Just about four weeks after the purchase, the driver's side door lock started acting strangely. The key fob wouldn't lock the driver's side door, even after replacing the battery and trying the spare key fob. Interestingly, using the key blade from the fob allowed me to lock and unlock the doors. However, both the key fob and the master switch on the driver's side failed to lock the driver's side door, although they worked fine for the other doors and hatch. Additionally, the car horn no longer sounded after locking the door twice with the key fob.

Despite trying various troubleshooting steps, including replacing the door actuator with a new part from Mazda, borrowing a friend's driver's side door master switch, and swapping out the fuse twice, the issue persisted. Online research led me to suspect the rear BCM (body control module) might be faulty, but testing disproved this theory.

In a last-ditch effort, I downloaded and installed Forscan scanning software and purchased a Forscan scanner OBDC reader from Amazon. Surprisingly, it worked, and I was able to retrieve error codes: B1172 and B126A. After researching these codes, I discovered that B1172 indicates a circuit short to ground in the rear BCM, while B126A relates to a symptom with the driver's door interior lock.

I apologize for the lengthy explanation, but I wanted to provide a detailed account of my troubleshooting efforts in the hopes of assisting others facing similar issues. If anyone has insight into what these codes mean before I resort to taking the CX-5 to a shop, I would greatly appreciate it.

Thank you in advance!

Jat
I have the same issue with my wife's 2016 Mazda Cx-5 sport. The driver's side door doesn't lock using the key fob or unlock. I have to put the key in to open the door. All the other doors lock and unclock with the key fob. I used to think it was the actuator and had once asked my mechanic to take a look at it. But just got busy and now just got used to it.

I can only open the rear hatch, if the driver side door is unlocked with the key.
 
That's so weird. With all the car jacking going on, it's unsafe for my daughter, which is single mom driving with an unlock door. I have one more trick to try before taking it to the dealership for deeper dive. I will buy an used rear BCM and give that a try.

Thanks for sharing your story as well.
 
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I have the same issue with my wife's 2016 Mazda Cx-5 sport. The driver's side door doesn't lock using the key fob or unlock. I have to put the key in to open the door. All the other doors lock and unclock with the key fob. I used to think it was the actuator and had once asked my mechanic to take a look at it. But just got busy and now just got used to it.

I can only open the rear hatch, if the driver side door is unlocked with the key.
By the way, it's not the actuator. I replaced a brand new actuator, which I bought from a local Mazda shop. I also went back to check my work after I saw code B1172 & B126A when I used the Forscan software.

However, I also had the Compustar remote starter installed at Best Buy. No issues for months after the installation though. Therefore, I am leaning toward the rear BCM at this time.
 
By the way, it's not the actuator. I replaced a brand new actuator, which I bought from a local Mazda shop. I also went back to check my work after I saw code B1172 & B126A when I used the Forscan software.

However, I also had the Compustar remote starter installed at Best Buy. No issues for months after the installation though. Therefore, I am leaning toward the rear BCM at this time.
RBCM even a used is quite expensive and you don’t know if it’s in good condition. Make sure to get one with warranty. I’ve seen a member from Russia fixed the burned-out RCBM by himself replaced some power transistor.

2 error codes:

DTC B126A:11 [REAR BODY CONTROL MODULE (RBCM)]
Front door lock-link switch (LH) lock circuit malfunction
With the front door lock-link switch (LH) unlocked, the rear body control module (RBCM) detects a short to ground in the front door lock-link switch (LH) lock circuit.


DTC B1172:11 [REAR BODY CONTROL MODULE (RBCM)]
Front door lock-link switch (LH) unlock circuit malfunction
With the front door lock-link switch (LH) locked, the rear body control module (RBCM) detects a short to ground in the front door lock-link switch (LH) unlock circuit.


Both system malfunction locations point to front door lock-link switch (LH). Have you checked the door lock switch or replace it with a new one? It has to be much cheaper than a RBCM. And a reprogramming by the Mazda computer (dealer) is required to the new / used RBCM.
 
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I’ve seen some relatively cheap used ones on eBay but FSM shows it needs to be programmed for vehicles with native Mazda OBD tool.. (or 3rd party programmer if anyone knows?)
 
RBCM even a used is quite expensive and you don’t know if it’s in good condition. Make sure to get one with warranty. I’ve seen a member from Russia fixed the burned-out RCBM by himself replaced some power transistor.

2 error codes:

DTC B126A:11 [REAR BODY CONTROL MODULE (RBCM)]
Front door lock-link switch (LH) lock circuit malfunction
With the front door lock-link switch (LH) unlocked, the rear body control module (RBCM) detects a short to ground in the front door lock-link switch (LH) lock circuit.


DTC B1172:11 [REAR BODY CONTROL MODULE (RBCM)]
Front door lock-link switch (LH) unlock circuit malfunction
With the front door lock-link switch (LH) locked, the rear body control module (RBCM) detects a short to ground in the front door lock-link switch (LH) unlock circuit.


Both system malfunction locations point to front door lock-link switch (LH). Have you checked the door lock switch or replace it with a new one? It has to be much cheaper than a RBCM. And a reprogramming by the Mazda computer (dealer) is required to the new / used RBCM.

Hello yrwei52,
I'm new to Mazda CX-5 vehicle. So when I google for the door lock switch, it came up with the master window and door lock switch. Stupid question, is the door lock switch is the same as the master door lock switches mounted on the driver side door? If you're referring to the master window & lock switch on the driver side, yes, I have swapped out with a known working master switch from my friend 2016 CX-5 and still the same issue.

Thank you for your feedback!
 
I’ve seen some relatively cheap used ones on eBay but FSM shows it needs to be programmed for vehicles with native Mazda OBD tool.. (or 3rd party programmer if anyone knows?)
I do see some for less than $50 with 30 days return warranty. I was able to dump the configurations of my current rear BCM with the forscan tool that I have now. I just want to see if the replacement rear BCM fixed the locking issue before I do any programming.

Thanks!
 
Well, got a new master window switch for the driver side and that's not it. Same ole issue with the lock & unlock. Rear BCM is the only reason for the lock on the driver door doesn't work at this time.
 
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