I started my 2000 chevy blazer this a.m. It remained started for 20 minutes. When I put it in reverse it shut off. It cranks but will not turn over. It have sat all day an does the same thing. My service engine light comes on. It was checked about two weeks ago with a trouble code P0171.

Fuel injectors ? Are there being commanded on by the PCM/ECM ? Do you know how to test fuel system , fuel pump relay etc... Fuel pressure test ! Follow this guy's testing procedure ! GM Fuel Pump Relay Testing If you can't your best bet is take to the dealer !

first you should make sure the fuelpump is working properly.you will hear a slight buzz when turning key on without cranking.if not its defective... happened to me with my 1996 blazer...if its good then check fuelfilter

It Was the electric fuel pump on my blazer when that happened and also replaced fuel filter.

Check for fuel pressure and check for spark during the no start condition. Also check the crank sensor, it is located behind the crank pulley mounted in the engine block on the 4.3 L engine
I hope this helps.

If you have injectors where carburetor was, watch injectors when it dies to see if they shut off before or after it quits.

Have someone sit inside and turn key on, the fuel pump in the fuel tank should make a humming noise for a few seconds then shut off (primes the fuel system) until you start cranking.

Wow, you seemed to have changed what I would have recommended to someone in your shoes.

If the fuel pump relay is hot, I think that there could be a crossed wire somewhere.

I would take it to an auto electric shop to diagnose problem if it's worth putting more money into it.

Have replaced starter, alternator , ignition wiring, neutral safety starter switch, still happens ??????? Do you know how to do automotive electrical testing ? Not guessing an replacing parts . Not starting ??? How ? when you turn the key to start does the starter motor engage an spin the engine ? Or when you turn the key to start nothing happens ? A no crank no start or a cranks no start . Plus you shoud learn how the system works before trying to diagnose .

Just having fuel is not enough on this engine, here are the spec's. Even 1 psi out of spec can cause a no start.

Fuel Pressure
Pressure 1 (PSI) Pressure 2 (PSI) Pressure 3 (PSI) Range Note Range Note Range Note 60.0 - 66.0 Y131 50.0 - 63.0 Y132 66.0 Y133 Y131: System pressure with ignition on (PSI).
Y132: System pressure while idling (PSI).
Y133: Fuel pump pressure (PSI) with return line briefly restricted.

Replace the nuetral safety switch it about 36 dollars at orielly auto parts