I know there are hundreds of threads out there for this but nothing seemed to help me.
96 351 Windsor. Everything was fine until last night when I got done changing my coolant. I flushed it, replaced thermostat and filled it back up with 50/50. About 2 hours later is when I got the P0171 code. It seems to be running fine but I'd like to know where to try and start to fix it.

 

Just clear it for now. If it comes back, then look for a problem. The system may have purged residual air within,- triggering the DTC.

 

I cleared it with my Scan Gauge last night then it came back on this morning. I can try clearing it again.

 

Look for vacuum leaks.

 

I think I might have found the problem. The hose that's going from the air tube to the filler neck on the valve cover was off. I forgot to put it back on. When I got in the truck and checked my Scan Gauge it was showing P0171 and P0174. I cleared them and started the truck and drove it stopping a few places and doing an ignition cycle and nothing's come up so far.