Code PO455 has been detected. My check enging light came on. on 2004 GMC Envoy

What can I check for the cod PO455? I changed the gas cap, light went off and the came back on. Was told to put vasoline around the seal in gas cap.

2 answers
Assumming you purchased the gas cap from the dealer here is another posibilty.

Customer Concern: Evaporative (EVAP) DTC P0455
Tests/Procedures: 1. Start and run the engine.

2. Activate the purge solenoid using a scanner and check for vacuum through the solenoid.

3. Activate the vent solenoid and apply 2 inches of vacuum to the fuel tank and check Fuel Tank Pressure (FTP) sensor voltage. It should go up above 1.3-1.7 volts and hold steady.

4. Perform TSB #05-06-04-014A. It is possible that a fracture as small as 0.508 mm (0.020 in) may be found in the fuel tank filler neck, causing a DTC P0440, P0455 and/or P0442 to set. Due to the location above the liquid fuel level, this small fracture will not cause a visible fluid leak.
Potential Causes: Filler Neck
Fuel Tank Pressure (FTP) Sensor
Gas Cap
Purge Solenoid
Vent Control Solenoid
Wiring Connection

Diagnostic Codes: P0455
Potential Causes:Filler Neck,Fuel Tank Pressure,(FTP) Sensor,Gas Cap,Purge Solenoid,Vent Control Solenoid,Wiring Connection. There are numerous test that would need to be ran to pinpoint exact problem. Cheapest route would be to get a new gas cap from a GM dealer, aftermarket caps usually do not work and vasoline is a wives tale. There are numerous confirmed fixes:OEM Fuel Filler Cap,Fuel Filler Neck,EVAP Canister Vent Hose,Evaporative Emissions (EVAP) Canister Purge Solenoid/Valve,Fuel Tank,Vent Control Solenoid. As I said without performing proper test the only other option is to replace part by part, but I would definetly start with GM gas cap.