I know trust me I know nothing either I'm just trying to guess at things. Idk of the truck will throw a CEL every time the relief valve relieves the pressure since that is what it's designed for. If I were going to go the route of checking things I would start with the cheapest thing and possibly eliminate them one by one. I'm not saying do that, I'm just saying if I were going to do that that would be the route I took.
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