atomztwins

Just picked up an '03 SE with 110,000 on the clock. I'm the 3rd owner, and the car has a good carfax (regular maintenance, not wrecked, etc). The car was taken care of very well, runs very well, and gets 28-29 mpg on the highway. It is a Cali spec car and is throwing a 420 code. The problem is, that I'm getting a preignition lifter ticking at low RPMs. Any help would be appreciated. Thanks in advance.