There are several reasons your Toyota Camry could be producing high NOx. The most common is a malfunctioning EGR system. EGR stands for Exhaust Gas Recirculation; and that is exactly what your Toyota Camry's EGR valve does.
The EGR system recirculates burned up exhaust gases back into the combustion chambers. Since the exhaust gas has already been in the combustion chambers once, it's burned most of its fuel and is now almost inert. This keeps the chamber temperatures down and thus reduces NOx emissions. (NOx is produced when chamber temps reach over 1200f)
Your Toyota's EGR valve should be inspected to ensure its proper operation. A working valve should be able to open its passage using manifold vacuum. All EGR systems are prone to collecting carbon build-up. Toyota specifcially uses a Transducer Valve EGR system which is very susceptible to being plugged with carbon deposits from the exhaust manifold. Once the transducer valve gets plugged up it stops the flow of vacuum to the EGR valve. Once vacuum is cut off, your Toyota is unable to open the EGR valve... and NOx emissions are increased.
