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Discussion Starter #1 (Edited)
2012 Mazda 3 Body Control Module Replacement.

The following post outlines how to change a BCM on a 2010-2013 Mazda 3.

Original Problem: My power locks were broken. When using my key fob the doors would not lock when outside the car. The fob could unlock the doors, but not lock them. Even more confusing; if I manually locked all the doors, then stepped outside and pressed lock on the fob, lights would flash, horn would beep, and the car was locked. Odd...

I after doing some research, I found that a handful of other Mazda owners had similar problems and remedy it by replacing the BCM. I purchased my Mazda 3 used and when I did, the seller actually informed me the last time they went to the dealership there were error codes associated with the BCM. I am fortunate enough to have a good friend who is a mechanic and I was able to use his shops OBDII scanner to verify the error was within the BCM. Sure enough it was!

I called my local stealership and they said a replacement BCM was ~$550! Not to mention labor at $120/hr for a job that would take ~2-3hrs. Not happening lmao! For $800 I can keep locking my doors manually. So to the forums I went...

While looking up the parts and procedure I would need to replace the BCM, I came across as post on this forum. https://mazda3revolution.com/forums/2010-2013-mazda-3-skyactiv-discussion/175178-body-control-module-bcm-compatibility.html . This post was super helpful because as @SuperBacon pointed out, the part did not “need” to be reprogrammed and could be hot swapped. Furthermore, the numbers on the BCM did not "need" to be identical. Meaning salvaged units should work - win! This however is where the internet and the dealership disagree...

I called two different Mazda dealerships to check if the part needs to be reprogrammed. Both service reps assured me the "BCM will need to be reprogrammed in order to ensure proper function". They did however say you can hot swap the part yourself and come in later to reprogram it, BUT, you may risk losing features like rain sensing wipers and power locks. One saving grace is that the dealership does not need to access to the BCM in order to do the reprogramming, so I was able to put all my trim back after installation. According to the technicians I spoke with, the reprogramming should take between 20 Min-1 Hour. *Side Note: I have made an appointment with my local dealership and I will update the post with final labor costs after my visit*.

The remainder of the post will outline the steps I took to replace my BCM.
The post will use a combination of links to Service Manuals, Personal Photos, and Tips and Tricks. My goal with the post is to make it easier for the next person to replace their BCM and save them the trouble of finding everything independently. Hope this helps!

Body Interior Service Manual Overview:
Mazda 3 Service Manual - Body Interior - Body

Interior Panel Removal Video:
This video shows how to remove majority of the interior paneling for the 2010-2013 Mazda 3. Although the video quality is poor, I found it helpful to see the direction to pull for removing clips.

Ebay: Terryville Auto - Salvage BCM
https://www.ebay.ca/usr/terryvilleautoparts
Seller was Terryville Auto. Part was exactly as described, shipped quickly and arrived within 2-3 weeks.


I paid $134.90 for the BCM and $17.50 to have it shipped.
Total part cost: $152


(Yellow writing is new BCM, Silver X and metal bracket is old BCM. Note that numbers do not match exactly)

Steps for replacing the BCM

1. Disconnect the Negative battery Terminal:

2. Remove the Front Scuff Plate:
Mazda 3 Service Manual - Front Scuff Plate Removal/Installation - Body Interior


3. Remove Front Side Trim:
Mazda 3 Service Manual - Front Side Trim Removal/Installation - Body Interior


4. Remove Hood Release Lever:
Mazda 3 Service Manual - Hood Latch And Release Lever Removal/Installation - Doors/Lids & Hood/Trunk

  • Be careful not to damage the release cable with the Flat Head screwdriver. I wrapped some painters tape over the tip and that worked fine
Moving over to the Center Console...

5. Remove Center Console Cup Holder:
The video is unnecessary, literally just pull up on it.


6. Remove the Shift Panel:
Mazda 3 Service Manual - Shift Panel Removal/Installation - Body Interior


  • I was able to get enough clearance to remove the required trim without disconnecting the cigarette lighter and heated seated functions.
7. Remove Center Console:
Mazda 3 Service Manual - Console Removal/Installation - Body Interior

  • Although it was tight, I did not disconnect the center console cigarette lighter or aux cord - I was able to slide the unit far enough back to get at the clips for the Lower Panel without removing them.
  • There are 4 screws which hold the center console in place. 2 Under the HVAC nobs and 2 inside the center storage compartment. (8mm Socket)
8. Remove Side Wall:
Mazda 3 Service Manual - Side Wall Removal/Installation - Body Interior


9. Remove Lower Panel:
Mazda 3 Service Manual - Lower Panel Removal/Installation - Body Interior

  • Removing the lower panel is tricky because it is secured by the center console. I was however able to get the lower panel off without completely removing the center console. By sliding the console back, I had enough clearance to pull the right side of the lower panel off.
  • Also make sure to be careful when removing the TC connector and any extra features from the lower panel.
Removing the BCM…

Mazda 3 Service Manual - Body Control Module (Bcm) Removal/Installation - Data Network

This step was easily the biggest pain. The module is located in a tight spot behind the steering wheel and is surrounded by wiring harness's. Nimble fingers are key for the next steps!


Looks like fun doesn't it...

10. Undo the Red Locking Tab


  • In order to release the locking tab, I used a small Alan key to press the tab down while prying out with my fingers.
(Optional) Remove Fuse Box:

  • In order to get access to the BCM and the connectors , I had to unbolt the fuse box and set it off to the side. (10mm Socket)
11. Disconnect the connectors:

  • To make sure that I was re connecting the correct wire harnesses, I marked all of them with a silver sharpie. The length of the wiring made it pretty easy to tell which went where but it was a nice precautionary measure.
12. Removing the 2 Metal Brackets:



  • In order to get the BCM out, I had to pull off the flat lower metal bracket first. This was done so I had enough space to get the full unit out. When re-installing the new piece I ran into the same problem. To install the new BCM, I attached the rear metal bracket (The one that bolts into the frame) first. Then positioned the BCM inside so it was almost in place. Next I slid the lower metal bracket onto the back of the unit. It is like a mini game of tetris and I found this the most efficient.
Old BCM:


New BCM:


Installation:

Re installing everything was WAY quicker than taking it out! Just follow all of the aforementioned steps in reverse. Make sure to triple check you put back all of the screws and bolts in the order they were removed. Some areas you lose access too if you skip steps.

Before installing the new part, make sure to spray the connectors and BCM ports with an Electrical Component Cleaner. This will ensure that any residue or debris which may have accumulated on the pins is removed for better contact. (Read the update section for more info/why this is important).

I was able to install the new BCM with no issues* and no reprogramming. However, as I mentioned above, Mazda says otherwise.

Ultimately I achieved what I was looking for. My key fob now works perfectly and can both lock and unlock my car!

I will update the post if I have any issues/once I visit the dealership and get final labor cost.

UPDATES

*So after driving the car for a couple days I ran into a problem... For some reason my signal lights and hazard lights decided to stop working. This was problematic for obvious reasons.

At first I assumed that this must just be an issue with the new BCM. It was a used part, so I was expecting some side effects for not reprogramming right away. What was odd was that the next day, I was driving around my neighborhood testing other features in the car to see if anything else was wrong and the lights started working again. Confused, I kept driving for a bit and then they turned off. At this point, I wasn't sure what could be the problem. So to the forums I went...

My google searches for Broken Turn Signal brought up a whole ton of results about fixing the turn signal switch, replacing fuses, changing bulbs, etc. I looked into all of them but none really made sense for my situation. I called my mechanic and explained what happend, he suggested that I just put my old BCM back in until I could get to the dealership and re program it. He did point out that the BCM should not just lose its memory though so he was confused why they would have gone on and off. After more research I narrowed down my problem to either a bad connection or a bad BCM.

The Fix:

I realized that during the installation process, I never cleaned out the connectors or the new BCM. The unit arrived fairly clean and I assumed that it was good to go. I watched a couple videos about how to clean electrical connections and decided to try it before putting the old unit back in. I dont know why I didn't think of this in the first place. You should always clean parts when you are installing them - Duh.

  • It is in my (not a mechanic) opinion, that for BCM connectors you do not need to use Di-Electric grease before re attaching them. Based off my research: Di-Electric grease can spread between the metal contact points where its insulting properties may reduce the connection between pins. Because the BCM is located inside the cabin, it will not experience enough moisture and contaminants to benefit from the sealant. Furthermore, the voltage from the BCM is not powerful enough to warrant protection from arcing or need extra insulation.
I bought some Electrical Component cleaner from Canadian Tire ($7) and used it to spray the inside of every BCM port and wiring connector.

https://www.canadiantire.ca/en/pdp/motomaster-electrical-contact-cleaner-150-g-0381740p.html

After waiting for the solution to evaporate, I plugged everything back in, re connected the -ve battery to test and, voila! Signals and hazards were back!

Hindsight is always 20/20... I should have cleaned the parts the first time I was installing them. Hopefully by reading this you wont make the same mistake!

As for the dealership... I have still yet to go. With everything working in my car, I see no need to take it to them right now. However, I will update this post if anything changes!

Thanks for reading! I hope this guide makes it a little easier to replace your Body Control Module!

Car: 2012 Mazda 3 6spd MTX Hatch 2.0L SkyActive

I would like to conclude this post by emphasizing that I am in no way a mechanic nor received a green light from Mazda about doing this. The following guide is just a documentation of the steps I took and resources I used to replace mine.

Cheers! - Ryan
 

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Excellent write-up Ryan!

Although I don't have to replace my BCM, I have removed some of those items before to route wiring or other things, but the links to the service manual areas can help. For me it was trial and error to remove some of those items, just figuring out how to remove them. I'm a do-it-yourself kind of guy, so this is something I would do, if it ever came up.

For the BCM, it seems like it is programmed to the car it was previously in, hence the re-program by the dealer.
 

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Thanks for that RyanGoesFast. Looks like you got lucky with not losing any noticeable functionality. The BCM replacement procedure has this warning.

 

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Discussion Starter #6
Hey Dynamho,

Yah when I spoke to multiple Mazda dealerships they also advised that I may lose some functionality by not reprogramming it right away. Thankfully they said it should not be anything critical (to engine or transmission at least) because the BCM controls mostly interior and automatic functions. As Arathol pointed out, things like automatic rain sensing wipers and automatic night lights, etc. I did experience some issues with my turn signals (read update for more) but fixed it by cleaning out the connectors with Electrical Connection Cleaner. Thanks for the heads up though!
 
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