oxygen sensor can throw low fuel codes,

start with fuel pressure, with a brand name gauge.
low fuel pressure is clogged line, crap in the tank, fuel pump going bad.
as far as the main relay goes, it will cause a misfire on all cylinders, not just 1 or 2. the relay simply turns on the fuel pump and sends signal to the ecu. solder joins can cause auto tansmisions to go crazy a bit.
once you can say that the injectors are getting proper fuel, move to the wiring. make sure the injecotrs are pulsing.
if not, ohm out the injecot wires(both ground and power from ecu)(ohm between the ecu and injector plug its self, then between the shocktower plug and ecu, shocktower plug to injector plug). check the ground plug on the drivers side.
once you are satisfied with fuel, be sure you are getting proer spark.
Spark trouble shooting