How hard is it to replace the lower intake manifold gasket? And what parts store on 2000 Ford Explorer

2000 Ford Explorer with a little over 114 thousand miles codes tell me tune up PVC x2 & injector O Rings also. We went cross country bout 5000 Miles with different fuel stops (fuel additives) check engine came on right afterwards. I put in 3 rounds of fuel injection cleaners.

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1 answer
HAVE ENG CODES SCANNED AND POST SO WE CAN ADV