Theory Of Operation

The Fuel Level Sensor information is a bussed message from the Body Control Module (BCM) to the Powertrain Control Module (PCM). The intent of the Fuel Level Rationality  is to check for a "stuck" fuel level reading. This is done to keep high or low fuel levels that are stuck from disabling OBD monitors. If a fault is set, the fuel level will default to a value that will allow the OBD diagnostics to run. 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. The PCM compares the difference between the fuel level value at the start of the diagnostic and the fuel level value after a calibrated amount of fuel consumption should have occurred. If the difference is less than a calibrated threshold, the test fails. Once a decision is made, a new diagnostic test will start.

The PCM also determines if the signal circuit has an open or shorted condition. The primary sensor is powered by a voltage close to vehicle battery (minus a diode voltage drop). 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 1.0 volt (full tank) and 10.5 volts (empty tank). The 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. The voltage reading for the scan tool is scaled  to read between approximately 0.6 volts (full tank) and 4.5 volts (empty tank) on a normally functioning system.