Oil Pressure Switch

Check that there is continuity between the oil press switch terminal and ground with the engine off.
Check that there is no continuity between the terminal and ground with the engine running.
If operation is not as specified, replace the switch.

Fig 1: Checking Continuity Between Terminal And Ground
Courtesy of KIA MOTORS AMERICA, INC.
Connect the oil pressure switch wire harness.
Turn the ignition switch ON. Check that the warning lamp lights up. If the warning lamp doesn't light, test the wire harness.

Fig 2: Connecting Oil Pressure Switch Wire Harness
Courtesy of KIA MOTORS AMERICA, INC.