2003 GMC Sierra 1500 P0455

I have a check engine light that has been coming on & off for about 2 years now.
The code is PO455 I have had it in two shops & all parts related to this code
replaced, some more than once. It has been smoke tested a number of time showing no leaks. It seems that when I fill up with gas, the light goes off and within 100 miles , the light comes back on once in a while. Do you think flashing the computer will get the light to stop coming on.

No, flashing the computer is a sign that the guy is desperate and/or clueless

A word to the wise . . . that code doesn’t specifically mean you have a leak

It means a sufficient vacuum couldn’t be generated and maintained in the evap system

It could be a leak, a purge valve which cannot draw a sufficient vacuum, or several other things

If it is the purge valve, a smoke test will not reveal it to be the problem

I feel whoever has been working on your truck doesn’t understand how these codes are generated, and how to verify that the evap system is working properly

Agree with @db4690

Even if I give him the benefit of the doubt regarding his skills… Your mechanic may not want to replace a cheap simple sensor that is a bear to reach. There is a sensor on top of the gas tank that measures evap pressure. If that hasn’t been replaced, it may be the problem. It is in the top of the fuel tank. You need to drop it to reach the sensor.

Flashing the computer will not help, I bought a gas cap from the dealer to cure that code for me, also cleaned where the gas cap meets the filler.

The pressure sensor has been replace two time, once by the shop and once by me. These are new AC Delco parts.

New gas cap from dealer.