Have a 2002 saturn sl2 sometimes when i start it and go into reverse it kills then i try to start it again i have to crank the key a few times before it starts again then it starts.. would it be a o2 sensor or intake air temp sensor?

Sounds like a possible short circuit

Have you tried cycling the key a few(3-5) times, before cranking(and trying to start it?
What i mean is turn your key on, then off, and on, then off(3-5 times), and then turn the key back on, and try to crank/start the vehicle, See what that does!

If that works then you have a fuel leak somewhere.

if that DOES'NT work, fuel is leaking into your engine while it sits
(more than likely bad fuel injectors!)

yes but I would try the fuel filter first

OUCH! sounds like by reversing the polarity the "good Samaritan" blew out your ECM. Unless you can replace it yourself you might want to call a place that will tow it for free to fix it

Clean battery connections and try moving the shifter to the neutral shifter position to start if it's an automatic.this would check the safety neutral switch.but it is most likely the starter motor.
Last,if you see an anti theft light on the dash when this problem occurs,leave the key in the on position for 10 mins and try starting again.

fuel filter dirty,weak spark plugs ,check the gap incorrect gapping will cause same hope this helps

This problem usually doesn't show up on diagnostic, it is a problem that saturn has with many of it's cars, it is either the ignition module or the crank sensor, I think replacing the crank sensor would be your best bet, that is what was wrong with mine and I didn't find that out until I replaced the fuel filter, the fuel pump, the ignition module and the coils, trust me, go with the crank sensor.

yes take it back if you are still under warranty.

check fuel pump

Try the spare key, the key itself might have unaligned notches, this same thing happend to me, and the barrel itself might be fine