Is the MAP sensor bad ??????

[BLACK BETTY '01]

I just plugged in my '01 Excersion to a programmer. The programmer reads EGTs, Tranny temp and Boost through the OBDII port. The problem I'm having is it reads 0-5 psi boost, and the turbo is just -a- screamin. I was told the that the ECM gets it's boost readings through the MAP sensor. Is this right??

Could this also affect my MPG????

Thanks