Excellent question. It is quite possible that your Hyundai Accent's (defective) knock sensor is causing high NO emissions. The knock sensor is responsible for telling your Hyundai's ECU (engine control unit) whether or not to retard the ignition timing.
A defective knock sensor may very well command the ECU to not retard timing enough or not at all. Without the ignition timing retarded to the correct degree engine emissions may result in high NOx; due to an increase in combustion temperatures. NOx is created when engine combustion chamber temperatures rise above 2500f. Advanced timing (or timing which is not properly retarded) will cause increased engine temps and result in increased NO emissions.
So go ahead with the knock sensor replacement if in fact your smog repair technician is certain the the knock sensor is faulty. More than likely NO emissions will return to normal once this repair is made, hopefully to passing results.
