Does your vehicale run sluggish or adnormally hot? Then it could be a clogged catalyic converter. Happened to me once and threw off just about every code in the PCM.
You may have to go to the dealership for 2 reasons, if your blazer is under 70,000 miles then it still has the emmisions warrenty. Also if you decide to fix it yourself, the codes will have to be reset with a special diagnostic tool that costs about $1500.

 

ok i got a used ignition switch and put it in last night and how all the codes are gone except
P0131
P0144
P0507

as far as the cat goes, ya it was plugged so about 2 weeks ago i took all the crap out of it. i still have the high idle though and i need to bring it down is there some way to test the sensors for the correct voltage.