Police in Cornwall are investigating thefts of catalytic converters from vehicles around the county.
Officers said there had been six incidents in Truro within a six-week period and drivers affected faced a repair bill of about £1,000.
Vans and 4X4s have been targeted due to their higher road clearance.
Police said they were investigating whether the stolen exhaust systems were being sold on or being taken for scrap metal value.
Insp Mark Richards from Devon and Cornwall Police said the scrap angle was being investigated because the devices contained "very small amounts of some precious metals, including platinum".
He said vehicles in St Austell, Camborne and Falmouth had also been targeted.
Driver of vehicles containing the converters have been advised to park in garages or well-lit areas.