Failing smog d15 civic?

hi i bought a civic from a friend. its a 1994 lx with a d15. it ped smog a few months ago but failed today. badly. compression is good, sparkplugs and air filter is new. no vacuum leaks as the idle is a little high 930 rpm. faild for high nox 881ppm and high HC p was 48 i got a 67. 243000k miles with a new headgasket and timing belt and machined block and head. not burning oil and cat is flowing fine. is it possible the cat is fouled or the sensors are bad? any thoughts?