I have a 2006 GMC Sierra C1500 pickup that has been to a GMC dealer for service on three occasions in the past 4 months for a check engine light with codes P0442 and P0455. The dealer replaced a sensor near the gas tank on the first visit. A month later to the day the light came back on with the same codes. The dealer could not find any problems and updated to software in the computer. I purchased a code reader and over the next month I had two additional check engine light and codes P0442 and P0455. I cleared these codes but took notice that they seemed to come on following a heavy rain. Last week after a heavy rain the check engine light came on again, and I took it to the dealer. He again could find no problems and once again updated the software. The dealer advised me that they filled the gas tank and ran the vehicle for over 40 miles. They kept the vehicle and ran it the next day for over 20 miles. They advised me that a smoke test was run with no problems and that I could pickup the vehicle. Upon starting the vehicle the check engine light was once again lit. A heavy rain accrued over night. The dealer advised that they can't find a problem. Can you help?

Do you have an OEM gas cap, or an after market? Most likely causes are a gas cap not sealing properly, or a leak in the EVAP system. Check all hoses and connections between the gas tank and the carbon canister.


P0442

Description
EVAP SYSTEM (SMALL LEAK DETECTED)


Evaporative (EVAP) emission system includes following
components: fuel tank, EVAP vent solenoid, fuel tank pressure sensor,
fuel pipes and hoses, vapor lines, fuel cap, EVAP emission canister,
purge lines, and EVAP purge solenoid.


Symptoms
You likely won't notice any drivability problems.


Causes

• A loose or defective gas cap
• A non-conforming gas cap
• A small leak/hole in a fuel vapor hose/tube
• Other small leak in EVAP system
• damaged vent o-ring seal

Solutions

• Remove and reinstall the gas cap, clear the codes, and drive for a day and see if the codes come back.
• Install a new gas cap
• Inspect the EVAP system for damage in the hoses. Repair is needed

It is an OEM gas cap

the trucks have issues with the vent valves getting trash in the valve and randomly sticking shut or open enough to cause those codes. napa had a filter relocation kit for my old mans to put the filter over the trans. sometimes you can get by cleaning it and soaking it with wd40 but it will come back in a year or two if you dont relocate it.

I had same on buick lasabre off and on for three years. Replaced purge valve solonoid and fixed it.