Vacuum Decay Test
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
If the weak vacuum test passes, the PCM will command OFF the EVAP purge valve, sealing the EVAP system. The PCM tests for vacuum decay in the EVAP system by monitoring the Fuel Tank Pressure (FTP) sensor for a change in voltage over a period of time. If the decay rate is more than a calibrated value, a DTC P0442 (A small leak of 0.02-0.06" somewhere in the system) will set. To diagnose, go to SELF-DIAGNOSTICS - 3.4L ALERO, GRAND AM, IMPALA & MONTE CARLO article.