Hi, my mom has a 2003 Ford Mustang, about 41,000 miles on it and everything is standard. She barely drives it that often except to work so there isn't much wear and tear on it. On the way home today she said she heard a loud pop and then the car started to run very loud. Two guys told here it was a catalytic converter that got blown. When she got home, i took it for a drive, it is pretty loud but quiets a little bit around 4th gear. then the engine light came on and when I looked at it, it said error po171 bank 1. I don't know exactly what the problem is and was wondering if anyone can help me out here.

 

http://engine-codes.com/p0171_ford.html

Start off with looking for a vacuum leak somewhere, then clean the air flow meter.

 

Loud exhaust and P0171 would make me think the 02 sensor is the culprit. Also, check the exhaust in general to see if there is a leak, specifically before the O2 sensor.