Mazda engine check light

My 1999 Mazda Miata check ingine light is on, with 62k miles on vehicle. Dealer says that it is do to bad gas. He describes it as like creasote in chimney. The code 402 pops up on computer and they say they have to remove the head to clean at $300.I have tried a store bought fuel system cleaner to no luck but out $10. Is this the only alternative I have or would maybe an induction clean solve my delima. Thanks