Ok this is the scenario, i had this 1991 volvo 740 s/w failed nox at 15 mph at 2066ppm max 785ppm hc 36ppm max 114ppm co 0.04 max .73% at 25 mph it passed nox at 704ppm max 724ppm hc 22ppm max 88ppm co 0.01% max .61% and check engine on with codes 231 lambda a... Login to read more.