The introduction of the three way catalyst (TWC) has considerably reduced emissions from gasoline fueled cars. Most of the pollutants from these cars are emitted during the cold start period. At lower ambient temperatures (below +20°C), as in the Northern part of Europe, these emissions increase considerably. In Sweden engine block heaters, mostly in combination with compartment heaters, are often used to increase the driver's comfort. The increased engine temperature also reduces emissions. Block heaters are also suitable for after-market installations, and can thus reduce emissions from in-use cars. This paper reports the investigations of the effects of a block heater on emissions and fuel consumption. It was decided to conduct tests on three types of light duty vehicles: cars with TWC, cars without TWC, and one car with very low emissions (i.e. TLEV). The cars were tested according to the FTP-75 test procedure at +22, +5 and -15°C ambient temperature.The test results showed substantial reductions of emissions for all cars with a block heater, at low ambient temperature. The reduction of CO and HC emissions in FTP-75 can be more than 80% for some cars. The relative reduction was less for older cars without a catalyst, but the absolute reduction was about the same as for TWC cars. NOx emissions were less affected and even increased marginally in some cases. The results also showed considerable reductions in particulate emissions at the lowest ambient temperature. The fuel consumption for the vehicle and the energy consumed by the block heater was measured and it was found that the preheat time for the heater could be optimized to minimize the total energy consumption. Some further optimizations and future development of this technology were also discussed.