Most modern cars have an oil pressure sensor that monitors the oil pressure in the engine. If the oil pressure drops too low, the sensor will send a signal to the engine control unit (ECU), which will turn on the check engine light.
If you disconnect the oil pressure sensor, the ECU will not be able to monitor the oil pressure, and the check engine light will come on. This is because the ECU does not know whether the oil pressure is too low or if it is just because the sensor is disconnected.
