It's obvious what they're doing - they're ROBBING YOU! The correct sequence is to figure out what is causing the problem, prove that you are right, then apply the fix, and if necessary install new parts. Your dealer started at the 'install new parts' end. Now that they have been proven stupid and incompetent, they want your to pay more, pay more, pay more.
Tell them to refund they money you have already paid because THEY FAILED TO FIX THE PROBLEM!
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator