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P0455 means a large leak, and large leaks are usually very easy to find (smoke comes out like a chain smoker in a bar). Most of the time, it is the cap is off or not even close to being closed.
However, here is a bulletin. Show it to the techs and have them pull up the fix.

Service Engine Soon (SES) Light On, Diagnostic Trouble Code(s) (DTC) P0442 Small Evaporative Emissions (EVAP) Leak and/or P0455 Large EVAP Leak (Reposition Fuel Filler Housing Seal) #05-06-04-055 - (Sep 2, 2005)
Service Engine Soon (SES) Light On, Diagnostic Trouble Code(s) (DTC) P0442 Small Evaporative Emissions (EVAP) Leak and/or P0455 Large EVAP Leak (Reposition Fuel Filler Housing Seal)
2006 HUMMER H3

Condition
Some customers may comment that the SES light is on.

Diagnosis with a scan tool may reveal a P0442 and/or a P0455 EVAP Leak DTC.

Cause
The seal between the filler housing and the fuel filler tube may be restricting the fuel cap from sealing correctly.


What happens is when they install the tool, it seals ok so nothing is picked up with the smoker.

Quick and easy repair, about ten minutes.