Sir, the thing is that its a 3-cyllinder car, so cng is not recomended,CNG would destroy its engine,u might have noticed the change engine noise after cng installation. Secondly it sometimes show engine light ON, which indicates that rpm is not adjust...coz some times in city(vario) it happens.
This is an underpower car, i personally prefer Vitz uptill 2004-model. Coz they are 4 cyllinder cars.
Take-care and drive safe.