Theory Of Operation

This diagnostic is run to verify the Oil Pressure Sensor has a valid zero pressure point (offset check). The oil pressure offset reading is essentially the voltage output of the sensor at atmospheric pressure.

Two assumptions are made for this rationality:

1. The oil pressure will always bleed down to atmospheric pressure within a certain time period after the engine has stopped.

2. The Oil Pressure Sensor will have a stable output voltage after key-on and before the vehicle starts to crank.

After a calibrated amount of engine off time; enough time to allow the Oil Pressure Sensor to stabilize (bleed down to atmospheric pressure), the Oil Pressure Sensor offset test will be performed at the next key-on. A reading of the Oil Pressure Sensor output is taken when the ignition is turned to the "on" position before engine cranking. Once power is applied to the sensor a delay time of approximately 20ms is required before the sensor is powered up and can output a valid reading.

At atmospheric pressure a "good" pressure sensor will output a voltage within an upper and lower voltage limit. The lower threshold is greater than zero volts so a circuit check can be performed on the device. The reason there is a span between the limits is because there is a small amount of air behind the diaphragm that varies in volume based on atmospheric pressure. If it is determined the sensor voltage is outside of these boundaries for a calibrated amount of time, the sensor is invalid and a failure will set.