What causes a 2001 Ford Focus to shut off when stopped?

Update: My daughter has a 2001 Ford Focus. It started shutting off every time she came to a stop yesterday. Today the engine will turn over but won't turn all the way on. We replaced the plugs, thermostat, throttle body, idle air sensor, throttle position sensor, mass air flow sensor, and put in a brand new battery.... show more My daughter has a 2001 Ford Focus. It started shutting off every time she came to a stop yesterday. Today the engine will turn over but won't turn all the way on. We replaced the plugs, thermostat, throttle body, idle air sensor, throttle position sensor, mass air flow sensor, and put in a brand new battery. What else could the problem be? Had a mechanic look at it and he gave some suggestions but he's not 100% sure. We're going to try those but I wanted some other opinions also.
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