Emission Monitors are technically not repairable. Emission monitors are internal self-tests which the computer performs to make sure the entire emission systems is functioning properly. In order to pass the smog test, the check engine light needs to be off and the monitors must be READY or COMPLETE. To get the monitors ready requires driving your Nissan truck for 100-200 miles and over the course of a few days; in both city and highway traffic conditions. The truck should not have an illuminated check engine light and all emission systems need to be functioning properly. If both these conditions are true, the emission monitors will become ready for the smog test.
One caveat to making the monitors READY or COMPLETE is that if the original emission fault was not fixed before resetting the check engine light and clearing the codes, the fault will trigger the check engine back on before or during one of the emission monitor self-test. The solution is to have a smog check repair mechanic diagnose your Nissan King Cab's smog check failure. The repair has to be made properly, the computer reset, the truck be driven for a few days (and at least 100 miles), and then obtain a smog check retest.
Do not obtain a smog check with the check engine light on; your Nissan King Cab will fail the smog retest. An experienced smog check repair technician should be able to extract the trouble code(s) and and use that information to repair the fault which is causing the light to illuminate.
As far as having erased monitors... it is very possible that while the auto repair mechanic attempted to repair your truck (replace the oxygen sensor) they cleared the existing trouble code(s) in the computer, as well as the "emission monitors" (which is required and standard protocol).
