how can i fix my problem please please help me?

i have just bought a vauxhall astra estate 2000 v reg 1.7 ydt the problem i am having is that it wont rev past 3000 rpm and wont go past 80 mph but engine does not miss or sound like there is out up with it i have had it put on the computer yesterday and there was a fault code to replace air mass sensor which i have done a brand new one put it on and it is doing the same do i need to take it back to get it reset or should it still work i have also changed the the fuel filters in the tank and under the bonnet what the hell is wrong with it.