1998 Cadillac Deville check gas cap

The evap system(fuel) could have a large leak, it assumes it could be a fuel cap loose or off. Possibilities include the fuel tank pressure sensor, faulty vent solenoid or purge solenoid, or a physical leak somewhere in the fuel system. Periodically the ECM commands the vent solenoid closed, cycles the purge solenoid to apply vaccum to the fuel system, and monitors the rate of decay. If the decay is deemed rapid, it will set a large leak fault.