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Problem Clearing P0455 code

ccmobsccmobs Member Posts: 1
edited October 2020 in Chevrolet
Hello...I am desperately trying to get my 2004 Chevy Aveo P0455 (Lg evap leak) code fixed. I took my car to a qualified shop with mechanics I know, and they could not find any leaks. They did try swapping out gas caps, but that didn't work. I replaced the evap canister and the purge valve (both located in rear passage well), and that did not work.

I am getting a weird noise and a bump sound from the gas tank area--BUT only when I first start the car, not afterwards, and I believe this started at the same time I got the first check engine light (P0455 code). Can anyone please offer some ideas of what this could be? I am now thinking there is something going on with the lines going from the tank to the evap canister. Please see video here:


  • thecardoc3thecardoc3 Member Posts: 5,747
    The sound you are hearing is just the engine controller testing the circuit for the cannister vent valve and is normal.

    Evaporative system diagnostics require that you know exactly how the test that caused the system to generate the code is run. The large leak test is performed by turning on the cannister purge valve to pull a vacuum on the system and the cannister vent valve to seal it. Then the computer watches the pressure in the fuel tank and cannister and expects to see more than 8" of water vacuum be achieved. If you are watching this with a volt meter atmospheric pressure is approximately 1.6v and the voltage should rise as the vacuum increases to about 3v in order to pass the test.

    With a full function scan tool GM has a bi-directional command called Purge and Seal for doing evaporative emissions testing that allows the technician to run the test exactly the same way the computer does. In your car's case the system isn't able to build or report the vacuum so the test fails so the technician needs to command the test to run and then start blocking pieces off to see if the system can be made to build a vacuum.

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