496 Mag HO threw a high water pressure alarm. A mechanical water pressure gauge reads normal. Discovered a broken wire on the water pressure sensor connector and repaired it. The smartcraft gauge continues to read 40.1 PSI even with the engine off. Replaced the sensor with no change. Engine has 40 PSI with the engine running or not. Normal readings on the mechanical gauge. Cleared the fault history with a code reader and reset the smartcraft to no avail.

Are there two water pressure sensors?

You might check what the resistance is at the sensor with the engine off. Disconnect the cable and run the engine and measure the resistance of the sensor when running. There may be a short or open in the harness.

That was it. Broken 5 Volt sensor power splice in the harness.

Hi I am having the EXACT same problem with my boat. Can you tell me where you found the broken wire? I seem to have power at the sensor I need to double check the ground again. Any help would be wonderful!!!

The problem was a break somewhere in the wire harness. It was the line that supplied 5 Volts to the pressure sensor at the output of the seawater pump. It was not possible to find as the wire harness is too complex and goes everywhere. We ended up running an external wire from the PCM connector to the sensor connector. You need the wiring diagram in the service manual to determine the pin number at the PCM. The sensor connector itself was a difficult repair using solder and epoxy as you can't just buy a new connector. It comes with a new harness. F that.

This same problem however could be caused by a defective pressure sensor. Start there. I suspect in my case too much stress was placed on the connector and wire when an impeller change was performed by the marina.