Q: Is it normal for the P0455 code to always come up when engine is on? on 2009 Pontiac G8

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In the past 2 years this code always shows up when I take the car to the dealer. About a year ago they found what they called a major problem, so far they have replaced the solenoid valve, rear evap canister, and left axle seal and also the fuel cap. Now a year later and the code is popping up again and they are telling me thesmoke test is negative. I feel like I purchased a lemon and need advice. Thanks
(3) Answers
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Apparently they are not very useful; Have them run an EVAP test with the Tech2 and watch the Fuel Tank Pressure Sensor operation. There's not a leak per say, just the computer recognizes a leak if the pressure sensor is not reading properly.
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P0455 is a large leak, or also known as a weak vacuum test failure. One of the first tests the system runs is to open the purge valve so that the engine can draw a vacuum on the system, and it closes the vent valve so that the canister and tank pull into a vacuum. The vacuum level has to exceed 8" in water to pass this first test. Prof is correct about using Tech II to command the system to run this step with the bi-directional controls and monitor the fuel tank pressure sensor output. I've had the purge valves stick on these and not draw any vapors and so it can't go into a vacuum and the test fails.
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