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Had the infamous P0455 fault pop up. Replaced vent and purge valve and checked the lines for cracks / holes. Code still came up. Got an OEM gas cap and installed. Reset code and all was fine for two days. Started truck up today, BAM P0455 again. Not sure what else to check now since the three main causes have all been replaced already...
 

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The fuel tank pressure sensor is the input that can generate this code. The system only conducts this test 1 time during cold starts. The ecm is not seeing the right amount of vacuum during its test. This can be caused by multiple different things. A bad fuel tank pressure sensor, a cracked evap hose, a cracked fuel filler neck. This code can only be generated by the lack of evap system sealing, a lack of vacuum, and the perception of lack of vacuum. Search this #09-06-04-024. You have already hit the most common causes. I hate telling people to waste money but my next move would be a fuel tank pressure sensor If I didn't have a smoke machine to diagnose the actual root cause. hope this helps.
 

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what about putting a vacuum gauge in line? You would know if the sensor if working anyway.
Sensor wiring? Ground?
I'm sure you cleaned the top gas neck well. Was the Oring somewhat dry on the new cap? Silicone...
 
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