It has an internal leak letting water get into it.
You switch off the pump, drain the pressure tank, then remove old gauge and fit new one.
You will need a fuel pressure gauge. Connect the gauge to the fuel rail and start the engine. The gauge will tell you how much pressure they pump is putting out.
If you already change the unit and it still the same then it can be the oil pump.
there should be a pressure gauge on a pipe leaveing the pump, or if it dumps into a catch pan then watch the flow,
You will need a fuel pressure gauge.
With a fuel pressure gauge.
Defective gauge, defective oil pressure sending unit, low of oil, defective oil pump.
Pump broken, even if it seems to be running, OR, pump needs "priming" (get water into the pump since it will not suck air), OR something is blocking the water from exiting the pipe (ie, back pressure, as if "destination is already full") OR there is no water feeding the pump. OR, maybe the other side of the pipe is leaking, hence no "pressure"
Install a gauge in the plumbing. Usually 1/4" pipe thread. Or you can use a tire gauge and check the pressures on the Schrader valve on the tank. With the Pump on, you will be reading system pressure. With the pump off and all water pressure released from the system, you are reading tank pressure.
First, gauge connect with pressure line and return line. then turn on engine. you will see the rate of flow and pressure on gauge.
Well, pump, storage tank, pressure switch, pressure gauge, Cycle Stop Valve, tank tee, drop pipe, wire, (with a submersible pump) gate valve and possibly water filtration. bob...
Have to remove oil gauge sending unit and check pressure with a master gauge. Sender or gauge could be faulty. I would not suspect the pump unless you see signs. First of all, does the gauge have pressure when the oil is cold? If it does and the pressure drops off when the oil gets hot, then you cold have a pump or relief valve problem. Removing the pump and inspecting should be your last resort.