Ignition Problem!!

My roomate has a 2001 ford focus with 157.000 miles on it, the car was having problem to start the engine, but after a few days the car just died. It doesn't start the engine at all, u can hear the starter trying to start the engine but it looks like an ignition/electrical problem (because the lights on the car and dash are on, and battery volt. is 12.6). The only way to start the car is by jumping it. But it can't start the engine by itself, she told she had a sensor changed a few years ago, which resolved the problem. (she doesn't remember what sensor was it). Does anyone have had the same problem?....or know the location of this sensor?...I thought it could be the maf sensor..but i'm not sure...thanks for ur help.!

are the battery terminals clean? does it run fine once its jumped? with the meter leads accross the battery check what the voltage drops to when the car is cranked it should drop no more than 9v, or you can take the car to a parts place and have them check the battery,charging, and starting systems.

^ Yeah that! I would check the teminals for corrosion or looseness.