Friday 14th February 2025

If you’re here just to learn how to replace the brake switch for a Mazda CX-5, you can jump to the bottom. If you’re interested in all the symptoms that led up to me actually knowing a faulty brake switch was my problem, here is the tl;dr.

I was out driving my car and an unfamiliar (to me) dashboard warning light came on. It was the one of a car with skid marks behind it, turns out that is the Traction Control indicator. I had a few places to go that day, so parked and figured I’d sort it out later. But on my next drive all the warning lights were gone so I figured it was a false alarm. But then they came back on again, this time along with the check engine light. I parked at my destination and then a few minutes later when I tried to start my 2016 Mazda CX-5 to head home for the day the dreaded happened: it wouldn’t start. All the usual suspects went through my head; dead battery, electrical issue, flooded engine, etc. After a few minutes of reading the manual and trying a couple more time unsuccessfully to start it, out of nowhere it started.

When I got home I pulled out my trusty diagnostic scan tool and engine code reader. They are super easy to use and well worth keeping one on hand. (If you don’t have one, seriously get one. For just $20 it can be a life saver, if nothing else to arm yourself with knowledge when contacting your mechanic. I initially bought it to figure out why my car was failing smog: check codes not completed after battery disconnect. Anyway…)

The diagnostic code that came back on this day was “Brake Switch A/B Correlation”. After some Googling, the first thing I checked was my fuses, I checked any that related to braking or stopping. All seemed fine. I had no idea what a brake switch even was though, and after more reading was even more confused since it says if faulty it can affect brake lights, shifting, ABS, Traction Control, Cruise Control, etc. But now it made sense why the Traction Control (TSC) light was on since so many things can be affected. But also frightening was that it says a fault brake switch can lead to STALLING ENGINE WHILE BRAKING. Read all about the P0504 Brake Switch Error Code here: https://www.way.com/blog/fixing-the-brake-switch-error-code-p0504

After that I called my local Mazda dealer, they said it would be $180 to diagnose, plus parts and labor to replace if it indeed was a bad brake switch. I made an appointment for a couple days out, then went back to researching. Fwiw I also found out that my brake lights were only working intermittently.

In the end it all made sense: Brake switch goes bad and car doesn’t know that you are applying the brakes. If you are driving this is bad news, but if you are trying to start your car it’s equally bad: If the car doesn’t know your brakes are on, it doesn’t complete all the checks and won’t let you start your car, also if started it won’t let you put your car in gear.

After watching the below videos, I actually bought one online and replaced it myself, and of course cancelled my appointment that would have cost probably upwards of $400. (I actually bought two, first a cheap aftermarket one ($13) that arrived next day so I could try it, then an OEM one ($30)that took longer to ship but I felt better using long term).

Super easy to do, watch these two videos.

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