P0251 - fuel pressure target cannot be reached.

Either you have restrictions in the fuel flow (blocked filter, tank stack pipe or drawing in a bit of air from a bad seal for example) or your fuel pump is defective. Either way your fuel system is struggling to make the required fuel pressure. Have the other 2 injectors been compared to the new injectors? Unlikely to be a problem but worth doing a leak off comparison to make sure they're all roughly even.