•The system presented in this study can reduce NOx emissions in a Diesel-powered Euro VI Heavy Duty truck working at low engine regimes by up to 80%.•The system is composed by an electric gas heater (EGH) coupled to an automotive thermoelectric generator (ATEG).•Simulations show that the EGH-ATEG system can work energetically independent of the vehicle, so there is no extra consumption of fuel.•This system can improve SCR efficiency up to 55% during low engine regimes. Selective Catalytic Reduction systems (SCR) are very efficient on reducing NOx. However, they only perform properly when exhaust gas temperature is higher than 180 °C. This means that for low engine regimes combined with cold engine temperatures, SCR systems remains inactive. This study presents a new approach to minimize the amount of NOx emitted by diesel engines of Heavy-Duty Vehicles during low engine regimes and low exhaust gases temperature conditions. We propose the addition of an Automotive Thermoelectric Generator (ATEG) coupled to an electric Exhaust Gas Heater (EGH) to make the SCR system inject the urea solution at low engine regimes. This EGH-ATEG system, which can be retrofitted in any existing vehicle, is designed to be energetically closed, so there is no extra consumption of fuel. Experimental results show that NOx emissions can be reduced up to 80% when an EGH is added to a standard diesel-powered Euro VI Heavy Duty truck configuration. Apart from that, the use of an ATEG installed downstream of the aftertreatment system can produce the energy required by the EGH, which means that the EGH-ATEG system can work energetically autonomous and independent from the vehicle’s electrical system. This system can improve SCR efficiency up to 55% during low engine regimes.