How Not to Save Fuel in a Suburban!

The check engine light on my 99 Suburban came on when I was driving about 15 miles per hour for no apparent reason. Took it to my friendshouse where he put it on a scanner. He said the reason the light was coming on was because the engine was having to trim for extra fuel. He said it was probably a vacuum leak and ask if anything thing strange has happend to my vehicle. (Here is where it get's interesting!) I told him I put one of those magnetic things on my fuel line that supposed to save fuel by 11%. He said "that's the problem". I ask how this was the problem because it was guarenteed to work. He said it was working, it was leaning out the fuel just as advertised....but the computer was now over compensating and putting more fuel to the injectors! After the computer sees this for a couple of days it senses a problem and throws a code out. Oh, by the way he said the magnetic thing works on Fords and older cars.
Bottom line, you just can't save fuel by this method in a Suburban with a vortec.