Bought an 07 Mustang GT a few months ago. Ran fine until 3 weeks ago and it threw a code. Started with the P0191 but a couple days later turned into P0171 system too lean bank 1 and also P0301 cylinder 1 misfire. Replaced the fuel filter with a motorcraft fuel filter. Changed the FRP sensor. Replaced all plugs with new motorcraft spark plugs and changed all ignition coils as well. Did a smoke test, found a leak on the after market K&N intake. Tightened and closed off known leak. Drove perfectly fine for 2 days and then slowly starting building back up a misfire. After cleaning out fuel system and intake drove around again. No misfire at the time. Drove another 2 days and system slowly started building up misfire again and seems worse in traffic. I have no idea what else it could be right now and have been told it could be fuel injectors, fuel pump, or the cat converter. Any ideas would be greatly helpful as a local shop would be over a grand for any of these things.

 

Don't fire the parts cannon. Start with what you got.

Did you buy a Motorcraft or Brosche FRP sensor, or some cheap knockoff? Bosche is the OEM for the Mustang. What brand of ignition coils? Motorcraft or nothing. Generic sensors and ignition coils and other brands are KNOWN to fail on these cars. Yes even in as little as a few days.

FRP is probably fine if you just have the misfire. Start with the coils.

You can either swap #1 out from another cylinder and see if the misfire follows the coil or take one of your old ones and swap it in. If they're not Motorcraft, I wouldn't even do that. Replace them all with new Motorcraft ones.

If the misfire stays on the same cylinder, then take a look at the fuel injector. Again, Motorcraft of Bosch only, don't waste your time with generics or offbrand.
If you do replace the fuel injector, you don't need to replace them all unless you're doing a flow matched set. 8 new injectors that are not flow matched won't be any better than 7 older injectors and 1 new one.

 

I did buy some brand I’ve never heard of for the FRP sensor. Got it from advanced auto parts and it was the only one they had. Paid $100. Released the pressure and replaced it. Car seemed to run fine after that. But it was only for a day. When the pressure built back up and ran for a little while it started to build up the misfire again so I thought it was fixed until it wasn’t. I’ll look for a Bosch sensor and start there.