The oil pressure switch is located in the oil filter housing (the picture shows the oil filter of the 316i, 318i models). Unscrew bolt 020 and remove the oil filter cover so that the oil can drain into the engine.
Disconnect the electrical wire from the oil pressure switch and unscrew the oil pressure switch "12".
Screw the appropriate pressure gauge into the hole of the oil pressure switch.
Screw on the oil filter cap.
Start the engine and let it idle. The oil pressure should be about 0.5 bar.
Increase engine speed to 5000-6000 rpm. Oil pressure should be between 4.0 bar (diesel engine: 3.8 bar) and 4.7 bar.
Screw in the pressure switch with a new ring and tighten to a torque of 30 Nm.
Tighten the oil filter cap to a tightening torque of 30 Nm.
In case of deviations from the specified value, see the table for diagnosing faults in the lubrication system.