Granatelli MAF Sensor problem

Having problem with my 99 Suburban. I recently put on Flowmaster Exhaust system, Electric fans, K&N Air Intake and a Granatelli mass air flow sensor. Ever since then, the check engine light would come on for a week or so, go back off, etc. I finally had the codes checked and it indicated that both banks were running lean. Put the stock MAF sensor back on and have no problems. Has anyone else run into this problem?
I also tried the MAF sensor on my friends 96 GMC pickup and he is having a different problem. Kind of makes me wonder if I didn't get a bad unit. Granatelli is not being helpful at all and I don't know if I should just throw the unit away or what. Thanks for any input

A lot of people say the Granatelli is a waste of money unless you are running a supercharger

The aftermarket mass air flow sensors can be ordered two ways. The part number ending with a "c" is for vehicles with a cold air box such as your K&N. Also if you service your filter and put too much oil on it it can cause all kinds of drivability issues. GM even has a TSB for an excessively oiled aftermarket air filter and will not repair the fault if they determine it is the cause. Hope this helps.