The fuel tank pressure (FTP) sensor provides information about the differential pressure of the fuel tank relative to atmospheric pressure. This sensor is required for the powertrain control module (PCM) to diagnose the evaporative emission (EVAP) system. The diagnosis of the FTP sensor consists of functional range check, circuit continuity check, and rationality check of the measured fuel tank pressure.

Offset check

The signal offset test is used to detect a faulty FTP sensor that may be responding to changes in pressure, but have an offset in the signal output. Initially, the test occurs under conditions when the EVAP system has been isolated from the engine and is expected to have a reading near ambient pressure. This occurs when the EVAP canister purge valve has been closed and the EVAP canister vent shut valve is open, so that the expected differential pressure in the fuel tank system is near zero.

Offset error suspicion: The offset error is set if the absolute value of the fuel tank pressure is greater than calibrated value while EVAP canister purge valve is commanded closed. Once fault suspicion is set, then the purging phase should take place followed by a plausibility check.

Offset error plausibility check: The plausibility check compares diagnostic results before and after canister purging. Once fault suspicion is set, the purging phase will be triggered. The purging phase is followed by the plausibility check to confirm the fuel tank pressure offset fault. If the absolute difference between fuel tank pressure stored before and after purging phase is a calibrated value and the difference between fuel tank pressure stored before purging phase and its minimum stored value measured during canister purge phase is a calibrated value, an offset error is detected and the corresponding fault will be set.

Incremental check

The incremental check measures the change of the fuel tank pressure caused by the purge mass flow while purge is enabled. Under normal conditions while changing the mass flow through the EVAP canister purge valve, the fuel tank pressure will also change. A sensor that has become stuck in range will cause this test to fail. During evaluation period while the enable conditions are fulfilled, the absolute change in the fuel tank pressure is monitored. To complete the test, the change of the purge mass flow must have also been sufficient. If the purge mass flow has been changed sufficiently without causing the required pressure change, a signal stuck error is detected.

Oscillation check

The oscillation check is designed to detect a pressure signal that is abnormally noisy. This type of sensor noise could potentially cause an incorrect leak detection diagnosis or even prevent the leak detection monitor from running. The oscillation check measures the amplitude and periodicity of the fuel tank pressure signal within an allotted evaluation period. Under normal conditions, the measured pressure signal should be relatively stable with pressure fluctuations less than a calibrated threshold. With the start of the check, two timers will begin to run. The first timer represents the overall evaluation period. The second timer represents an observation window and measures the amount of time the relative pressure signal is within the allotted pressure amplitude. This timer will reset to zero each time the relative pressure is greater than a calibrated amplitude threshold. The relative pressure is formed from a comparison between the current fuel tank pressure and the pressure that was stored at the last time the second timer was reset. If the pressure signal continues to fluctuate for the entire evaluation period, means no positive results are achieved, then a signal noise error is detected.