Fidanza flywheel + check engine light

Last Friday, I had my mechanic install a Fidanza lightened flywheel along w/ an ACT clutch. I got the car back, and after about 10 seconds of driving I get a check engine light. I take the car back to the mechanic and it reads out as the crank sensor. I ask my mechanic what the deal is and he says that a lot of steel flywheels have a magnetic part in the flywheel that is used by the crank sensor to read the flywheel. As a result, the sensor is freaking out because the flywheel is made of aluminum rather than the stock steel flywheel. The car runs great but I've never heard of people having this problem. Any comments? Is my mechanic on crack and just forgot to reinstall the sensor? I'm trying to get ahold of Fidanza to see if they know anything.