retarding your timing reduces the combustion chamber temps. there fore reducing NOx. NOx is a big one now. the egr "exhaust-gas-recurculation" is just what that is for too, so checking the egr when you have high NOx is good.
if it is hydro carbons giving you problems, a fresh cat usually does the trick. i keep a cat in my shop just for every two years when i test my truck.
also i have bypassed the air injection control valve by running a hose straight from the air pump to the exhaust.
another thing i have found that helps is having fresh gas.
i have noticed that i test better with 87 octane gas. octane is the resistance to burn. if you have a high comp. engiune you will need high octane to jkeep from pinging but if you have lower compression engine you can run a lower octane fuel. if you run a higher octane fuel than needed it just doesn't burn as well.
