Trouble Code P0449 Definition: EVAP (Evaporative Emission System) purge/vent solenoid circuit malfunction.
1- Purge or vent solenoids defective
2- Loose EVAP system connectors and/or wiring
3- Failed EVAP vent solenoid
4- Failed Gas Cap
If you have replaced the gas cap once and P0449 came back this means there is an issue with the EVAP canister vent valve located near the EVAP canister and the fuel tank. P0449 typically indicates the solenoid on the EVAP canister vent valve is not receiving the proper voltage and or ECU signals.
Trouble code P0455 is related to:
1. A loose or improperly affixed gas cap
2. A non-conforming gas cap (i.e. not factory/original brand)
3. Other leak or damaged component in EVAP system
With a P0455, the most common repair is to:
Remove and reinstall the gas cap, clear the codes, and drive your Tahoe for 100-200 miles and see if the trouble codes reappear.