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Check Engine light on

I have 99 Merc. Grnd. Marq., code 0174 know showing up on the scanner, I also had 0171. I have done a vacum test and holding 21.5 Hg, I have change all rubber vacum hoses even if they did not need changed. I even changed the gromet for the PVC Vale on the right valve cover. Checked the injectors and all are clicking. I had changed spark plugs earlier and show a normal firing color. I had changed the fuel filter(last week) and MAF sensor a month ago. do I need to change the MAF sensor again. Lost?