Theory Of Operation

The Fuel Level Sensor information is a bussed message to the Powertrain Control Module (PCM) from the Body Control Module (BCM). The fuel level rationality will set a fault for a fuel level reading that does not change over an accumulated threshold of fuel consumption to keep stuck high or stuck low fuel levels from disabling OBD monitors. Since the amount of fuel consumed from trip to trip varies based on customer drive cycles, this diagnostic can accumulate fuel consumption over multiple drive cycles. The diagnostic will run until enough fuel has been consumed to make a decision. Once a decision is made, a new diagnostic test will start. The PCM compares the difference between the maximum fuel level value and the minimum fuel level value after a calibrated amount of fuel consumption should occurred. If the difference is less than a calibrated threshold, the test fails.

The PCM also determines if the signal circuit has an open or shorted condition. The BCM supplies 12.0 volts on the signal circuit through an internal pull up resistor. If disconnected, the signal circuit voltage will be equal to source voltage, which is approximately 12.0 volts. On a normally functioning system with no circuit faults, the voltage reading on the signal circuit at the Fuel Level Sensor should be between approximately 0.25 - 4.75 volts with the Fuel Level Sensor connector plugged in. The voltage varies based on the fuel level in the tank. The remaining voltage is used up through the resistor on the Fuel Level Sensor. Therefore, the voltage reading on the return circuit should be zero volts with the connector plugged in.