Experts have told 5 live Investigates that tens of thousands of drivers are breaking the law by driving diesel cars without crucial particulate filters. They're known as DPFs - or diesel particulate filters - and they stop harmful particles of soot from entering the atmosphere. But they get clogged up and can affect car performance. What's more, they can cost thousands of pounds to replace - so drivers have them removed. The programme also reveals how the MOT test which is designed to detect the removal of DPFs isn't fit for purpose.