How do you clear codes for emission test?

Nissan Altima 99, Recent spike in NOX emission test?

• The following are the emission tests for Nissan Altima 1999. 2006 ASM2525 Test, HC (ppm)=21, CO% =0.01, NO (ppm)=157, RPM=1811, Dilution=14.31 2008 ASM2525 Test,HC (ppm)=10, CO% =0.00, NO (ppm)=198, RPM=1614, Dilution=14.8 2010 ASM2525 Test,HC (ppm)=12, CO%= 0.00, NO (ppm)=136, RPM=1590, Dilution=15.0 2012 ASM2525 Test,HC (ppm)=3, CO%= 0.01, NO (ppm)=392, RPM=1708, Dilution=14.8 Moreover, I checked the EGR, it is moving well and functional when the engine RPM is raised(2000-3000). Can we exclude a faulty converter since HC and CO were always very low? Could the spike in NO of year 2012 Test is due to emission test shop has faulty equipment? I did push the EGR valve open with my fingers while car on park and the engine running. The engine went dead. Does this mean FOR SURE it is a faulty converter?

• Answer:

  What I notice is your 2006 21ppm HC and 157 ppm NO and as the car runs lean in 2012 HC 3 ppm and the NO rises to 392 ppm. I am thinking some injector cleaner. Lean running makes combustion temp to rise hot combustion makes more NOX. Properly functioning EGR cools combustion temp reach behind under the egr valve lift up the diaphram and see if the engine at idle stalls (oops you did that). Older engine more wear blow by or vacuum hoses may have splits and manifold air leaks? What are the O2 sensor voltages doing and are there any trouble codes? Remember some time a hot running motor is all it takes to make NOX high, proper coolant mixture put in more water if necessary.