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Hi Everyone,

I bought a 2018 Mazda 6 Signature with 21,000 miles on Sunday. I drove it over 100 miles back to my home with no problems and really enjoyed the car. On Monday morning I drove to do a few errands, when I got back in my car to go back home everything went wrong!

I began driving and noticed that the car started automatically applying the brakes for no reason (there was nothing ahead of me). On the instrument cluster, I saw the message, "Smart City Brake Support Malfunction." I immediately called the nearest Mazda dealership to have it serviced. I had to limp it all the way to the dealership (8 miles away), along the freeway the car sporadically braked and I was terrified someone was going to rear-end me. On top of all this, the AC stopped blowing cold air (when it had previously worked fine the same morning).

The fact that it had problems the day after I bought it has me worried. The car has a clean title, no accidents, and clean Carfax. It doesn't have any damage and everything else seems to be working.

Has anyone else had these problems? The used car dealership sold it to me "as is", but I have the Mazda manufacturer warranty and an extended warranty. Should I take the car back to the dealership I bought it from, even though I will have to pay over $1,000 restocking fee? What do I do, any advice? Maybe I am just having bad luck???
 

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Welcome to the forum!

Sorry to hear about your issues. Between the AC and the SCBS freaking out it sounds like either your ECU is going nuts or you've got an electrical gremlin somewhere. ECUs can be reset by the stealership or you can do a rudimentary one by leaving the battery unplugged for a little bit (15-ish minutes ought to do it).

Checking for electrical gremlins, however, can be much more difficult. First thing to do would be to check the connectors on your battery terminals and making sure they're secured tight and not corroded. Next, check your ground wires and anything in the engine bay to make sure it isn't frayed or exposed. Unfortunately, my workshop manual seems to be missing the ground locations for the engine bay, so I can't show you a proper diagram.
 

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Thanks for the response! Yeah the service department at Mazda had a feeling that the two problems were related. I'm hoping they will know what to do and will know to reset the ECU and such. I'm still shocked that all this happened literally less than 24hrs after I bought the dang car.
 

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Thanks for the response! Yeah the service department at Mazda had a feeling that the two problems were related. I'm hoping they will know what to do and will know to reset the ECU and such. I'm still shocked that all this happened literally less than 24hrs after I bought the dang car.
Let us know what they end up saying, and don't be afraid to come on here and ask/verify what they're telling you before making any decisions. Most of us aren't certified Mazda techs but we all have real-world experience you can't get from a book.

And unfortunately that car may have had an electrical quirk before you bought it and either the place you got it from knew that and sold it to you anyway or had no idea. Could be why someone got rid of it in the first place.
 

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We see this issue quite often at the dealership, usually its as simple as having the PCM and the FSC (front sensing camera) software updated. Another thing to make sure of is that both the camera at the top of the windshield is clean and so is the front emblem as the radar is sent out through the front emblem.
 

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Sorry about your troubles. Hoping they get resolved.

For future reference if anyone encounters a similar situation - rather than trying to limp the car with random auto braking actions (if that's what you're experiencing) try to disable the system until you can get the car serviced. At least then you should be able to drive without the chance of getting rear ended due to a random brake application.

I'm well aware this shouldn't happen and shouldn't have to be done, but the option is there to turn it off and it's safer to do so if it's causing an issue.
 

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Discussion Starter #7
Sorry about your troubles. Hoping they get resolved.

For future reference if anyone encounters a similar situation - rather than trying to limp the car with random auto braking actions (if that's what you're experiencing) try to disable the system until you can get the car serviced. At least then you should be able to drive without the chance of getting rear ended due to a random brake application.

I'm well aware this shouldn't happen and shouldn't have to be done, but the option is there to turn it off and it's safer to do so if it's causing an issue.
I actually tried disabling the SCBS in the dashboard and it said it was disabled, but didnt actually disable the system. The dealership said that it failed to disable the braking system because it was likely a computer or electrical issue and therefore could not communicate.
 

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Mazda service called me today and said that I had a bad PCM and that they were going to replace it. They were unsure if the PCM also caused the AC issue, but they assume that they are related.
 

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I actually tried disabling the SCBS in the dashboard and it said it was disabled, but didnt actually disable the system. The dealership said that it failed to disable the braking system because it was likely a computer or electrical issue and therefore could not communicate.
At least you tried then! Surprised it kept happening if it said it was disabled. Hope the part replacement fixes your issues.
 

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Mazda service called me today and said that I had a bad PCM and that they were going to replace it. They were unsure if the PCM also caused the AC issue, but they assume that they are related.
Good to hear they found the issue.
Interesting that my PCM also just failed and needed to be replaced. I have a 2018 6 Signature, too. I had similar SCBS malfunction and erratic behaviour before it became undriveable.
 

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I once tinkered with the air box and caused a air flow meter fault , but it affected scbs , affected the transmission also , it started shifting very harsh . Check engine light came on

stopped the car , remove the mod I did , restarted and all went back to normal

puzzled me and still does , why a simple air flow meter fault affects scbs and transmission also
 

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I once tinkered with the air box and caused a air flow meter fault , but it affected scbs , affected the transmission also , it started shifting very harsh . Check engine light came on

stopped the car , remove the mod I did , restarted and all went back to normal

puzzled me and still does , why a simple air flow meter fault affects scbs and transmission also
The air flow in the sky active engine is very calculated and very technical, the intake and air box are specially designed for the air to swirl once it enters the intake and it swirls the air as it’s entering the cylinders as well . The intake can also change the flow of the air allowing the intake to utilize both a long and short runner depending on how much your giving it. This is one key part of how mazda keeps the torque band consistent through the whole rpm range. Therefore even having the slightest crack in the air box or leaving it undone just a little can throw a cel and cause the car to run poorly.
 
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