Mitsubishi Repair: Mitsubishi Montero Sports, fuel pressure regulator, mitsubishi montero

Question
My car is a Mitsubishi Montero Sports 2002 AWD 3.5. My car starts fine in the morning and have a problem after running  and stopping the car. After that car will not start. Ignition clicks and motor works but the car would not start after next attempts it will start and stop instantly after a few attempts it will start popping (like crackers) a few times and stop on continuing starting it will start popping when pressing the accelerator down popping sound becomes loud and the car starts with a few  shakes once it starts it is fine and runs fine as if nothing has happened. I had this problem for sometime. In the past 6 months I got a tune up done, changed the oxygen sensor and gas filter. It stopped and again this problem is back again. I have the check engine light on I went to Pep boys and got it scanned these were the codes it showed up PO455, PO421, P0171, P0174. I have seen many mechanics and have no positive answers. Hope you can help me to resolve this problem.
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Answer
Mohamed,
All of the trouble codes that were pulled from your car's computer indicate a fault the O2 sensor threw due a lack of fuel in the exhast.  Sense you already replaced it (or both of them), the focus should turn to the fuel system.  You mentioned you replaced the fuel filter, which can help increase the flow of fuel to the engine, but there are several other components which can restrict the fuel flow.  Have the fuel pressure regulator and fuel pump inspected. Also have the fuel pressure in the line checked.  If there's even a slight drop in pressure, it will throw those codes.  A loosely fastened gas cap can throw all of those codes, so have our mechanic inspect that if they haven't already.
Good luck!