2002 Dodge Ram 1500 - Error Code

I opened the gas cap while the truck engine was running, and the 'check engine' light came on. The OBD code, P0442, displays that there is an
emission leak. I had the code reset by a neighbor with a OBDII computer, and it stayed off for about 4 days...and is now back on. What else could P0442 be? Anyone else experienced this?
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