sierra problem.

Hay guys ,one of my friend has 02 sierra with 72k miles on it that has some problem latelythe check engine is on and couple times it flashed. car ideal's really rugh, espessialy when it's cold. we took it to autozone to read the code and it says the car is runing too lean and some of cylinder has miss fireing.we took it to GM and first they couldn't find any thing and we took it some were else and they said it needs new manifold gasket and it need to clean and flow test the injectors.do you guys think the problem can be from that?what can be othere causes?
Thanks

A little more info would help a lot. What engine, how long has the problem been happening, does it miss all the time, what are the exact codes?

it's 5.3L vortec and it dose it all the time.it has been runing like this for couple month now.i don't know if i can get the cods,but i'll try.

My 98 GMC does about the same thing. Idles rough , throws the check engine light -flashing of that light is NOT good either. We changed the injector on the offending cylinder misfire (in my case it was #6) and its still doing it. It uses oil too - I will be doing at least an (lower) intake gasket . Its a 2 piece plenum , your local Napa should have the gaskets for both (In Fel-Pro now)Do NOT get an old Victor Gasket kit - they are not made with the updated (corrected) designs. Expect to pay around $100 for the gaskets. 1 for the upper gasket -squarish,preformed and rubbery type that fits in a groove . 1 is a o-ring that fits around the injector pack (seals to the upper plenum) and 1 that is a key shaped that is for the throttle body itself. Then the lower intake -2 pieces.
Get a good code analyzer on it to see what the code means.