The "check engine" light illuminated and the "gas cap loose" warning flashed on. Replacing the gas cap did NOT solve the problem. The shop that cleaned the sand out of my injectors told me that the analyzer indicated that it was the vapor return valve and that it was more of a nuisance indication. However the check engine light stayed on. After procrastinating for a year, I finally broke down and ordered the "dealer only" part and replaced it. NOTE: the new part was configured differently so that sand and contaminants cannot get into the valve as easily by using an extra long vent tube that is routed to a less hostile area under the truck. Now that all my fuel system issues have been dealt with I can now concentrate on the nightmare called the electrical system. Oh by the way, aren't Chevy trucks supposed to be "tough and reliable"?.....I don't see it!