What sensor are they referring to?

my 2001 blazer is tripping the P0440 code which is EVAP system malfunction. on the OBD-CODES website, it is saying: 'Check and/or replace the sensor'. what sensor are they referring to? im going to replace the purge valve just to be safe. then if the code comes back, im going to replace the vent canister valve in the rear of the vehicle. other than those two, i just want to know what sensor are they referring to so i can inspect and or replace it. thank you for your inputs :))