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Hey there, I'm new here and have looked at several threads about this code P0455. I have a 2007 Trailblazer LT with a 5.3 engine and 179,000 miles. I have replaced the fuel cap and Evap Canister (the one by the fuel tank). I cleared the code and it popped up again. It reads as a Evap Large leak. The other code I'm seeing is a P0455P ????? any help would be great. Thanks and have a great day
 

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First, a quick little lesson on P0455.

Code P0455 is very common, but in the case GM products in general, two particular causes of this code predominate, to the point where many Chevrolet owners refer to this code as “the dreaded gas cap code”.

Failed, or defective gas caps that allow fuel vapor pressure to escape from the EVAP system is a common cause of this code on Chevrolet applications, and while an OEM cap usually costs less than about $50, replacement of the cap is not guaranteed to provide a reliable, long-term solution.

GM gas caps suffer from inherent design flaws that seriously affect the useful life of a new cap. Note that since a visual inspection of a suspect gas cap will almost never reveal the nature of the failure, a smoke test is recommended to either confirm, or eliminate a defective gas cap as the cause of code P0455 on Chevrolet applications.

The other major cause of code P0455 on Chevrolet applications is the fact that liquid fuel enters the charcoal canister when the fuel tank is filled past the recommended level, which is when the fuel pump “clicks off” for the first time. When liquid fuel enters the charcoal canister, the compacted charcoal disintegrates, and some particles can migrate throughout the EVAP systems’ vacuum and fuel vapor hoses/lines.

Some charcoal particles may become stuck in either (or both) the EVAP vent and purge valves, which prevents these valve from closing thus allowing vapor pressure to escape into the atmosphere.
 

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First, a quick little lesson on P0455.

Code P0455 is very common, but in the case GM products in general, two particular causes of this code predominate, to the point where many Chevrolet owners refer to this code as “the dreaded gas cap code”.

Failed, or defective gas caps that allow fuel vapor pressure to escape from the EVAP system is a common cause of this code on Chevrolet applications, and while an OEM cap usually costs less than about $50, replacement of the cap is not guaranteed to provide a reliable, long-term solution.

GM gas caps suffer from inherent design flaws that seriously affect the useful life of a new cap. Note that since a visual inspection of a suspect gas cap will almost never reveal the nature of the failure, a smoke test is recommended to either confirm, or eliminate a defective gas cap as the cause of code P0455 on Chevrolet applications.

The other major cause of code P0455 on Chevrolet applications is the fact that liquid fuel enters the charcoal canister when the fuel tank is filled past the recommended level, which is when the fuel pump “clicks off” for the first time. When liquid fuel enters the charcoal canister, the compacted charcoal disintegrates, and some particles can migrate throughout the EVAP systems’ vacuum and fuel vapor hoses/lines.

Some charcoal particles may become stuck in either (or both) the EVAP vent and purge valves, which prevents these valve from closing thus allowing vapor pressure to escape into the atmosphere.
First, a quick little lesson on P0455.

Code P0455 is very common, but in the case GM products in general, two particular causes of this code predominate, to the point where many Chevrolet owners refer to this code as “the dreaded gas cap code”.

Failed, or defective gas caps that allow fuel vapor pressure to escape from the EVAP system is a common cause of this code on Chevrolet applications, and while an OEM cap usually costs less than about $50, replacement of the cap is not guaranteed to provide a reliable, long-term solution.

GM gas caps suffer from inherent design flaws that seriously affect the useful life of a new cap. Note that since a visual inspection of a suspect gas cap will almost never reveal the nature of the failure, a smoke test is recommended to either confirm, or eliminate a defective gas cap as the cause of code P0455 on Chevrolet applications.

The other major cause of code P0455 on Chevrolet applications is the fact that liquid fuel enters the charcoal canister when the fuel tank is filled past the recommended level, which is when the fuel pump “clicks off” for the first time. When liquid fuel enters the charcoal canister, the compacted charcoal disintegrates, and some particles can migrate throughout the EVAP systems’ vacuum and fuel vapor hoses/lines.

Some charcoal particles may become stuck in either (or both) the EVAP vent and purge valves, which prevents these valve from closing thus allowing vapor pressure to escape into the atmosphere.
Thank you for the information. Is there a diagram of the Evap system available somewhere ? I guess since the gas cap is new, the canister has been replaced and I still have a "LARGE EVAP LEAK" CODE" maybe I should go for the purge valve next. Thank you again, Jon
 

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EVAP "purge valve" takes "controlled vacuum" off the engine. It's mounted on the engine itself. When disconnected electrically, it should have no flow through either tube if it's sealing correctly. Same rules apply for the EVAP "vent valve" by the purge canister near the gas tank.

I test with either a hand "vacuum pump" gun to see if the EVAP lines are 100% sealed or use a radiator pressure tester with an gas cap adaptor.
Website reference: https://www.rockauto.com/en/moreinfo.php?pk=8882864&cc=1440260&jsn=572&_nck=Ekt6BqBF1pwlXdx0gYlm1U17wbyQURW/JtcimnkkjpPvCpOUBflZuT3ZK04fYhcuDBv1UV5hzXTcAoSfo35W1Gfvt6q4ZMWy80wjjG+jzBq3qQ+Ab9XJzehptMJ1mvOl5jhgi2WRXvz5dlVCuNSmJlXREHO0jFsGGgsK5THQTgZ+HCRuoAtyrTFehKlTl0uWi3Tv082EHAFfrAUygHkfj9lMS75kObheyIFcE1wXNWhtKzflwQPWdY+W7DLfbM7KbMmZtCLTsBqq77EScejOKiRMSR56UUYlynbnAGmfRJ+rJ9GXbbEXboFI6nGU6fmNjQ3c5cA4C5zPIsN+YzwN2esbEdDJpjhqWOzvbxncQ2iac+7Tqaf8u7n5yiVp4KtrxIqMKBX7+Og71/zu54xL3w==

Then with a very full gas tank see if a slight pressure (less than 1/4 pound of pressure) is held by the EVAP system vent valve with the ignition off. Scan tool will verify there is a "held" pressure with a value. If not either visually search for leaks in the "whole" system or have the car dealer do the smoke test to make it simple in finding the leak.
 

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Changed the Fuel cap, Vent canister and Purge valve. Check engine light is out and hopefully it will stay that way. Thanks to all for your assistance,
 
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