My wife couldn't use her key to start her CX-5 a day ago, but my key worked. This morning my key wouldn't start it -- this afternoon it could. For a while it seemed like if you locked the car with one key, you had to use the same key to unlock and to start it. With further testing, I'm not sure that's true.

Any ideas?

 

Dead or dying key fob batteries.

 

Try this, hold the tip of your remote to the start button, push the start button with your remote ( while you have your foot on the brake pedal) & see if the vehicle starts. If it does then you need to replace the battery in each remote, CR2025.

If it doesn't then you need to go see Mazda.