By Christin Ayers

HAYWARD (CBS SF) — Ever worry that you’re getting ripped off when you take your car in for service?

Well, you may have been if you’ve taken it to Firestone.

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Bonny Kyamugabirwa has been going to the Hayward Firestone on La Playa Drive for the past six years, not knowing the tire shop has been under investigation for fraud.

“I come here for my regular oil changes,” Kyamugabirwa said. “I don’t know enough to be savvy if they were overcharging me.”

State officials say the store has been overcharging a number of its customers. It started with a string of complaints here in 2013.

Bill Thomas of the California Bureau of Automobile Repair says over two years investigators discovered a disturbing trend.

Thomas said they found, “unnecessary services being sold to consumers, potential fraud, charging for services that weren’t performed, significant oversell.”

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But then the bureau got an anonymous tip from a former Firestone employee alleging that the fraud was happening not just here, but at Firestone shops statewide.

“We prepared undercover vehicles and conducted covert undercover operations at 30 Firestone locations throughout the state,” Thomas said.

The investigation found that of those 30 locations, 21 Firestones in the state engaged in fraudulent practices like over-charging, including seven locations in and around the Bay Area.

Now the attorney general has drafted an accusation against each Firestone store seeking to revoke their licenses to do business in California.

A spokesperson for Bridgestone, which owns Firestone, said in a statement: “Bridgestone immediately launched an extensive internal investigation and shared our findings with bar. Bridgestone subsequently implemented appropriate corrective actions.”

State officials say there is only one way to make sure you won’t get ripped off next.

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“Always ask questions if you’re not sure what they’re explaining to you,” Thomas said.