My new 09 Mazda 6i GT less than 800 miles had the red key light on since I got it. Read the book, it said to have dealer check it out, but it was running fine so I figured I would call them this week. BUT, yesterday after driving it 5 miles, it wouldn't start. All the lights gauge lights started blinking and headlights lights started to dim. I checked the battery, the indicator was green. I put another battery in jump configuration and everything looked normal again, so I jumpstarted it. Now it is running flawlessly and the red keylight is no longer on. I plan to take it in to dealer just to see what they think. Any thoughts?

 

I think slow car sales at the dealer might be the culprit.
If your car was kicking around on the lot for a while, the battery would slowly self discharge. Once a battery in a Mazda gets to the point where it won't deliver at least 12V on start-up, it will get the engine going OK, but the battery volts are insufficient to kick the alternator into life; you drive around on whatever residual the battery can give, until below 10.7 volts, the PCM just gives up the ghost and the car goes into the twilight zone mode. (You get exactly the same symptoms if your alternator drive belt/ tension are not right- even worse, this can cook out your alternator because the PCM cranks up the excitation in attempting to maintain the alternator output: the alternator gets a double dose of heat, since the belt slip translates into excess heat conducted into the bearings and rotor metal and the over-excitation translates into excess heat due to electrical heat dissipation in the rotor too).
Boost starting or recharging the battery is the cure for the initially low battery because as soon as the alternator gets more than 12v for excitation, it kicks in and the volts across the battery go to over 14 volts and your (technical) troubles are over. It is only a temporary fix for an alternator drive problem though, so don't mess around; get it fixed pronto.
The story doesn't end here though; it suggests that your dealer short changed you (and Mazda) on the pre-delivery checks which are absolutely mandatory, and if neglected or omitted can result in major troubles later on.
You should insist that he supervise the pre-delivery operations and furnish you with written evidence that the Mazda pre-delivery protocol was followed.