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Those 8 injectors should have been covered under warranty. The extended injector warranty is 7 years or 200K. The bulletin states 2001 and 2002, and some 2003 trucks. If your truck had the symptoms of the bulletin, it should have been covered.

After the injector replacement, did they, or you, change the oil? If not, that could be your current problem. Leaking injectors cause fuel to enter the crankcase. Fuel diluted oil will cause low oil pressure, and is described as a symptom in the bulletin. If the oil pressure is still low after the replacement, and it has fresh oil, damage was done, and the engine needs replacement, on GM's tab.

14 PSI at idle is not the problem. The problem is, the pressure was OK before the injector issue, and now it is low. Something happened to cause the low pressure, and the dealer should be looking for the cause.