There are also genuinely useful 'emissions-control defeat devices' that operate by allowing a vehicle's engine to operate outside government-imposed tailpipe emissions parameters. These government standards force factory engines to operate outside their most efficient range of operation. Either engine control units are reprogrammed to operate more efficiently, or sensors that influence the ECU's operation are modified or 'simulated' to cause it to operate in a more efficient manner. Oxygen sensor simulators allow fuel-economy reducing catalytic converters to be removed. Such devices are often sold for "off-road use only".