Alex Dimant, owner of Diamond Auto Sales in Alameda, says he's been hit four times in the past three months and each one costs between $500 and $1,500.
"They're very valuable because of the precious metal inside; I think it's platinum," Dimant said.
Island Auto Sales and Tony's Motor Care across the street have also been repeat victims. So Dimant decided to stake out his car lot.
"I know that officers were out there four times yesterday, or last night," Alameda Police Ofc. Amelia Mrak said.
But Dimant says he'd like to see them patrolling in the early morning hours when the crimes are happening. He says he pays between $20,000-$30,000 in sales tax each month.
"It goes to the county and the city, and we're not getting much for it," Dimant said.
"We address all crime in Alameda, it's not just depending on sales taxes or who's paying more, it's all crime in Alameda that's being looked into," Mrak said.
It's illegal to buy or sell a used catalytic converter in California, which is why police believe they're being sold for scrap metal.
The police say they'll try to do a better job, but Dimant says he'll continue guarding his car lot, just in case.
