Could be a lot of things. From what I've read around the 'tubes it sounds like its almost always a wiring issue (worn insulation, bad ground, melted wires). I'd start by inspecting the all wiring related to the fuel pump (and especially the FP relay) and work from there testing everything in the fuel system (I've heard a melted spark plug wire can throw this code too...)
I don't know how trustworthy this source is or how applicable it is to our stangs, but it sounds reasonable:
Diagnostic Trouble Code (DTC) 543 indicates a fuel pump secondary circuit failure between the B(+) supply and the Fuel Pump Monitor (FPM) connection to the power-to-pump circuit. 99 times out of 100, this DTC results in a no-start condition.
If the engine starts and runs, the it is a pretty good indication there is a problem with the Powertrain Control Module (PCM). NOTE: Due to the internal circuitry of the Powertrain Control Module (PCM), a left/front Heated Oxygen Sensor (HO2S) signal short to power could produce a Diagnostic Trouble Code (DTC) 542 or 543
codes on these cars start at number 11 , code 8 does not exist
173 R, C HEGO (HO2S) sensor fault/rich
but 173 shows the one of the O2s or both are stuck RICH .........doesnt absolutely mean the o2 is bad just means the car has reached its limit of adjustment and cannot switch to lean do to an overly rich condition .....
im having same problem idles and runs funny. same codes 543 and 542 and lean. i installed a new fuel pump every sensor under the hood trying to fix this except the tps to get the car to run right and both new o2 sensors still has same codes and runs odd. runs great at wot.... have not changed fuel pressure regulator. that's next and i have checked all the wires unless i over looked something. this car only has 60,000 miles on it its a 95 gt automatic 5.0 someone please help lol