OIL PRESSURE I replaced the oil flow sensor and still have low pressure. Could the gauge be bad or is it the oil pump?

replace oil pressure sender switch
change oil filter
then if no better
fit a sandwich plate oil pressure reader between oil filter and block
to get the engine oil pressure when running

Do you mean what causes low oil pressure? This is a very reliable engine for the most part. If the oil light is on or gauge is reading low, have the oil pressure sensor inspected for failure. These sensors do go bad.

The computing system on your vehicle has no safe mode for running low on oil . The only thing that will happen is the check gauges light will come on.
The oil pressure gauge is an analog device that is located in the IPC. The oil pressure gauge is operated directly by the oil pressure sensor circuit. As the engine oil pressure changes, the resistance of the sensor and the current flow in the oil pressure gauge changes. The current flow in the oil pressure gauge causes the needle to move in order to appropriately display the engine oil pressure. A sensor resistance of 1ohms will display as 0 kPa (0 PSI) and a resistance of 88 ohms will display as 80 kPa (80 PSI ). The IPC also monitors the oil pressure sensor circuit and interprets the current flow in this circuit in order to determine when the engine oil pressure is too low. If the oil pressure is too low, the check gauges tell tale will turn on. The oil pressure sender is located on the left side of the 2.2L. The 4.3L engine, oil pressure sender is located at the top left side, near the distributor/high voltage switch.
Try disconnecting the battery cables an hold them together for ten minutes , reset ECM !

Generally low oil pressure is due to a failing oil pump. If you have a mechanical gauge you can pull the electric oil pressure sensor and use that gauge there to verify if the pump is bad. If pressure is low on the mechanical pump then the pump is bad. If the mechanical pump reads good pressure then its the pressure sensor that is bad.

see this tips and fix it. God bless you
The oil pump supplies oil to lubricate your engine. If the oil pump is worn or is not turning, the engine will suffer a loss of oil pressure, which may result in engine damage or engine failure.
The first sign of trouble may be a low oil pressure warning light, a drop in the normal reading on you oil pressure gauge (if your car has one), or the appearance of ticking or clattering sounds from your engine.
As a rule, most engines only need about 10 PSI of oil pressure for every 1,000 RPM of engine speed. Oil pressure will read higher than normal when a cold engine is first started because the oil is thick. Oil pressure will gradually drop as the engine warms up and the oil thins out. So normal oil pressure on a warm engine cruising down the highway is typically 30 PSI up to 45 PSI.
SYMPTOMS OF OIL PUMP TROUBLE
The first thing you should do if any of these symptoms occur is to stop your car, turn off the engine, let it sit for a few minutes, then check the oil level on the dipstick. If the oil level is at or below the ADD line, add a quart of oil to bring the level back up to the full mark. Add as much oil as is needed to raise the level to the full mark. Then restart the engine. If the warning light remains on, or the oil pressure reading does not climb back up to its normal range, or the engine noise does not go away, you may have a bad oil pump.
The other possibilities include a bad oil pressure sending unit, or a problem with the oil pressure warning light circuit or oil pressure gauge.
OIL PRESSURE SENDING UNIT
If the engine is NOT making any unusual noises and seems to be running normally, and the oil level on the dipstick is FULL, but you are still getting a low oil pressure warning light or low gauge reading, the fault could be a bad oil pressure sending unit.
The oil pressure sending unit is mounted on the engine block. On some applications, there is a spring-loaded pressure-sensitive diaphragm with a switch inside the sending unit. This switch completes the circuit to the low oil pressure warning light if oil pressure drops below a certain threshold. The unit may stop working if the diaphragm inside fails, if the switch is stuck, if the small hole that allows oil to enter the sending unit becomes plugged, if there is a loose, corroded or broken wiring connector at the sending unit, or there is a fault in the wiring circuit between the sending unit and warming light.
On vehicles that have an oil pressure gauge (electronic, not mechanical), the oil pressure sending unit has a small rheostat inside that sends a variable voltage signal to the oil pressure gauge when the diaphragm moves. A worn spot on the rheostat or any of the other problems just described for the simple pressure-type oil pressure switches can cause a problem.
FORD'S FAKE OIL PRESSURE GAUGE
On many Ford vehicles that were built from 1980 through the 1990s, the oil pressure sending unit has two switches, a low pressure and a high pressure. These vehicles also have an oil pressure gauge, but the reading on the gauge is not a true indication of real oil pressure. As long as the pressure to the sending unit is between high and low, the gauge will read normal. If oil pressure drops and trips the low pressure switch, the dash gauge will now read low. Or, if oil pressure goes up and trips the high switch inside the sending unit, the dash gauge will read high. Consequently, don't rely on the oil pressure gauge for an accurate reading in these vehicles. It is only a gross indication if the oil pressure is low, normal or high.
OIL GAUGE PROBLEMS
If the engine is NOT making any unusual noises and seems to be running normally, the oil level on the dipstick is FULL, and you have replaced the oil pressure sending unit but are still getting a low oil pressure reading on the dash gauge, the fault could be in the wiring circuit between the sending unit and gauge, or the gauge itself might be bad.
Check the wiring connections on both ends as well as wiring continuity between the sending unit and gauge. If no wiring faults are found, hook up a pressure gauge directly to the oil pressure port on the engine and check oil pressure with the engine running. If the engine-mounted gauge shows normal oil pressure but the dash gauge is reading low, the problem is a bad dash gauge.
On the other hand, if the engine-mounted pressure gauge reads low and you have done all of the above, chances are the oil pump is worn, or it is not picking up enough oil because of a restriction or blockage in the pickup screen in the bottom of the crankcase.
OIL PUMP PICKUP PROBLEMS
The pickup tube has a screen on the end to prevent large chunks of anything bad that ends up in the crankcase from being sucked into the pump. But we are talking BIG chunks of debris, not normal wear particles or carbon or dust or other microscopic-sized abrasive particles that can cause pump wear over time.

don't rely on the oil pressure gauge in the dash, it's not a real oil pressure gauge. It's just a glorified "low oil pressure" lamp. It only has two readings - "normal" and "low", nothing else. The "sensor" in the block is actually just an oil pressure switch which you replaced.. which only has two states, on or off. If your gauge is fluctuating between "normal" and "low", it means either that the switch or wiring to the switch is bad, or that the oil pressure is so low that it barely makes the pressure switch turn on. If it's the latter, then it could be either low on oil... not! Or something clogged, such as oil pickup tube, filter or oil channels. Other causes for low oil pressure is a failing oil pump, or worn out journals or main bearings, which basically means the engine is worn out.
Hope this helps a little. Could get it checked at a shop with a mechanical pump gauge to see what your true pressure is. Thatll tell you if it's the switch light is bad

If there was no oil flow the engine would have packed up and you would have got error codes or the oil warning to come up. So it is possible that the oil sensor pressure switch is defective and so must be checked.
However is you still think there is low oil pressure this means that the engine must be overhauled. The reason so low pressure of oil can be fault in the oil pump to faulty - wearing of the engine .

if your oil pressure gauge is reading low, the sensor, instrument cluster gauge or the pump is bad. remove sensor and use an oil pressure test gauge. reading should be measured and verified against manufactures specs. if the pump is bad, damage to the engine will result.

need to find out the pressure the pump is putting out hook up a mechanical pressure gauge most use the sensor port to hook into and test pressure if pressure good than u may have a sludge problem where a flush might help-if pressure low possible clogging [sludge build up] of oil screen-[which a flush or back blow-compressed air -to clear or clean pick up tube and screen should help ]--or internal engine problem or oil pump bad

It means the sensor that detects your oil pressure has seen a situation where the oil pressure is too low and engine damage will result. Could be just a bad sensor, but better have it checked with a mechanical gauge to see exactly what the pressure is.