My wife's 2017 cx5 stranded her last week. She had gone thru the car wash and soon after the car started baking automatically, releasing, then baking, releasing until it showed and shut off. Then it wouldn't start. We towed it to a shop and it started and was driven off the rollback. Shop looked at it, there were no codes, couldn't get it to happen and so she picked it up.
Drove it tonight and she went thru a puddle and the same thing happened. Now she's stuck on the side of the road again.
Any ideas? Must be water getting into something, but what would make it shut down and not restart?

 

The normal cause, but not only one for this, is a bad braking system sensor.
I would also point out a bad battery on these vehicles can cause all kinds of issues with no faults, including the braking system.
If you have not replaced your 2017, then you should do so now.
It's also time to look for another shop with a certified Mazda tech.

 

Thanks. Battery has been replaced. She is in rural NY with no Mazda dealers close. The front radar sensor is out but I can't see how that would be impacted by the puddle. Will look into the brake sensor. Thanks

 

You should really read your owner's manual.
The front radar system controls the Smart braking System (SBS).Your vehicle is equipped with a radar sensor (front).

The following systems also use the radar sensor (front).

• Distance Recognition Support System (DRSS)
  
• Mazda Radar Cruise Control with Stop & Go function (MRCC with Stop & Go function)
  
• Smart Brake Support (SBS)
  

The radar sensor (front) functions by detecting the radio waves reflected off a vehicle ahead or an obstruction sent from the radar sensor.

The radar sensor (front) is mounted behind the front emblem.
If “Front Radar Sensor Blocked” is displayed in the multi-information display of the instrument cluster, clean the area around the radar sensor (front).

 

The shop did not correctly test your Mazda. They should not have reaped your Mazda based on your description without you signing a waiver. For one they should have simulated the condition by at least hosing down the first the front brakes and any sensor exposed part or other electrical connections ,test, and then the rear to simulate the condition creating the braking problem.

Replacing a battery does not mean the battery is good. But I don’t think that the battery is the cause of the braking report. But I recommend next time in a shop or go to a parts chain that offered free battery health check.

 

Not sure with your description of what was actually happening with the car, I have had issues with cars in car washes and it has been the oxygen sensor getting blocked with water from the external breather side and it causes the engine to run extremely rich bucking and kicking and it may cut out all together till it clears with heat from the exhaust. It's usually visible with a scantool looking at live data when the problem occurs.

 

The front radar sensor has been an,ongoing saga, but I don't think it is related here. Mine went bad, bought a used one, went to have it calibrated but they are apparently VIN-locked and so the calibration kept failing when the car's computer tried to match the sensor. So for about 1.5 years we have not had cruise or the smart brake features. Dealer wants 3k....this is such a rip-off.
I have one other attempt to pair the radar sensor before I am,going to just have to eat the 3k and then sell the car. Electronics are ruining the world!