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The Water Pump and Oil Pressure Gage should not be related, but the wire to the pressure switch, may have been disturbed in some way durning the water pump installation.
I have looked over the wiring diagram, and if it were a fuse problem, you would see several gages not functioning.
The IPC uses the engine oil sensor signal to determine the engine oil pressure. The oil pressure sensor signal circuit is hard-wired to the IPC. The engine oil pressure sensor signal to the IPC is a variable resistance ranging from 0-90 Ohms . A sensor resistance of 1 Ohms will display as 0 psi (0 kPa) and a resistance of 88 Ohms will display as 80 psi (550 kPa) . The Check Engine Oil Pressure indicator will be displayed in the message center if the IPC detects a low engine oil pressure.
At the rear of the engine on the left side of center Check that the wire going to the sensor is not damaged and is connected to the oil pressure sensor.
The wire you are looking for should be TAN with a White Tracer going to the oil pressure sensor.
I hope this information helps.