Hi, I have a 2000 chevy impala and when I start it, it wont fully turn over unless i press the gas pedal. After I let go of the gas pedal the car turns off. I replaced the alternator, and the battery. I need help!

 

I assume you mean 'won't start unless you step on the gas' or is it engine won't crank???

 

Sounds more like a fuel problem. Check fuel related items such as fuel filter, air filter, dirty injectors, etc.