First off, I've done a search utilizing "brake" as a key word and wasn't able to find a solution to the following issue. Based on what I've encountered, this may be an SCBS problem, which seems to be a common issue with these vehicles from what I've read thus far.

My wife has a 2015 CX5 Grand Touring with roughly 76K and a manual (non-electric) e-brake. Last night, it suddenly started braking above 5 mph and would not allow either of us to exceed that speed before nose diving. Reverse works fine with no issues. I've since checked the brake/rotors on the passenger side for both front and back. I could see no visible issues. The dealer replaced the rear pads last year, and I'll likely replace the front sometime this week.

I've done the following in an attempt to trouble shoot the issue based on what's in the owners manual and what I've read elsewhere.
Disconnected the battery overnight to reset the sensors --- No improvement
Cleaned windshield in front of laser (outside)---- No improvement
Turned off SCBS --- No improvement
Turned off TCS --- No improvement
Turned off DCS --- No improvement
Turned off all the above at one time -- No improvement

The digital read out only shows that it's due for the scheduled maintenance and a low tire pressure warning (we haven't reset it). I have not tried pulling any codes as of yet with a OBD2 reader.

For this year, I don't believe there are any more sensors in the front, but can't confirm. If there is, please let me know where.

I did notice that the area in front of the SCBS Laser does appear to be "foggy," but I cant' tell if it's possibly a "reflector" at the base (don't know what it is exactly) or if the windshield has dust particles in front of the SCBS.

I would appreciate any advice or if there is anything that I may have missed.

 

Just a quick update

I’ve since replaced the front rotors and pads for an unrelated issue, and connected an OBD2 to check for any codes. None were found as well as cleaning the front exterior in case there were any sensors that I could not find.

 

The braking behavior sounds like SCBS. I have never had SCBS activate, but my wife had a 2014 GT and described what she believed was SCBS initiations a couple of times, where the car automatically braked without her intervention. Once when approaching a high hedge at the edge of a parking space, and the other when, after a heavy rain, low overhanging rain soaked tree branches, about hood height activated SCBS. Never had it operate by itself with no obstructions, however.

 

The first time this occurred was after my wife came to a stop light after exiting a highway. After crawling around the car, I didn't notice any loose connections or if there was any damage from something being run over.

As a follow up, we've since towed it to a dealer and their diagnosis was a complete transmission failure. They sent a sub-par video basically indicating that the issue was repeatable without checking the transmission pan for debris and only cited that no errors were recorded in the PCM. They've since said that there is a bearing issue without doing any significant inspection on the car. Their basic analysis was that it was trying to downshift at around that speed.

I've gotten different opinions from other mechanics as to what it might be, with the predominate theme is that it's an issue with either the TCM or PCM.

Has anyone heard of this or had anything remotely close to this?

 

Interestingly, my son’s 2019 Signature showed CEL and TPMS errors and was limited to 2000 rpm; limp home mode

Dealer preliminary diagnosis is rodents chewing through battery wires, mass air flow meter wiring. If the master wiring harness needs to be replaced we are talking $5K

 

Abysmal diagnostics by the stealership, but also not at all unusual or surprising. They're ready, willing, and able to hammer you for 7K or so for a new tranny, based on that piddly amount of analysis. I wonder if they even pulled the transmission connector to examine it and the near wiring for issues - I'm guessing not. And at the very least, they should have taken an extra 10 minutes to drain/inspect the fluid (after first checking the level), and send an inspection camera up the drain port to have a look around inside the case for debris, or some other obvious issue. And, if nothing from that, ask if you wanted to pay for them to drop the tranny pan and pull the valve body to get a look at the solenoids, sensors, and wiring. Not very much time required for that either, but I guess they couldn't be bothered with any of that.


Can't understand that type of thinking either, because the computers do lots of self-testing, and IMO it would be highly unlikely for one them to have an issue without any code being set (or a comm code if one is not communicating). Case in point is the early Mazda 3, which has a history of systemic TCM issues, most likely a result of being mis-located in a high heat area. But in every M3 TCM case I've read, there was at least one transmission-related code set.


There are TSBs from Mazda on a couple of transmission issues, however both of them are documented to have one or more codes set, and neither have anything close to the symptoms that you're describing.

If this were mine to DIY, I'd have a few things to test and try, but I don't know if you're up for anything like that. If not, you're unfortunately at the mercy of all of these people who are throwing darts. It's appalling to me how few of these 'professionals' seem to have any interest in doing comprehensive diagnosis these days. Just keep hanging parts until they stumble upon something actually work, and then hand you the oversized bill for all of their fiddling around.

 

another update

m5xguy

I requested that they drop the pan, but they wouldn't do it...basically they didn't want to spend any more time on it. I wound up calling Mazda customer service and all they really did was put me on a conference call with the tech at which point nothing was resolved. I did take the vehicle back and to an independent mechanic. They were able to replicate the problem, and basically said the issue was a result of a design flaw in the transmission case. They cited that this was an issue with another car they repaired at some point in the past. They pulled the information from other years that had recalls for the same problem. Unfortunately this year doesn't have a recall. The case itself is roughly $1800, but no one in the US has a new one at the moment with everything being on back order. I know the TC also has a recall on different years and this may be a issue with the car as well. The independent mechanic never saw any codes set either.

I agree that most are willing to just through darts/money at the issue and are unwilling to perform an actual diagnosis.

Does anyone know how to reach a Mazda Factory rep for the North Texas area? I need to discuss this with them.