Section 115D(1)(a) in The Central Motor Vehicles Rules, 1989
(a)the vehicle intended for retro-fitment complies with following conditions, namely:-(i)it conforms to Bharat Stage-II or subsequent emission norms;(ii)it belongs to category M1 or category M2 or category N1 with Gross Vehicle Weight not exceeding 3500 kg.;(iii)it is fueled by either gasoline or diesel fuel;(iv)it was not retrofitted earlier;(v)it shall not be fueled by any other alternate fuel;