Investigators found a stolen Honda Accord parked near Thornton Road and West Avenue in Merced on Wednesday.
Inside, they discovered 33-year-old Raymond Loewen and 35-year-old Sarah Smith from Atwater.
Deputies also found tools used for stealing catalytic converters inside the car.
Loewen and Smith were detained.
Officials say they've seen an uptick in catalytic converter thefts within Merced County over the last several years.
Authorities are urging people to park their cars in a garage when they can or to have their converters welded into the car frame to help avoid thefts.