YOU SHOULD NOT HAVE TO MESS WITH THE TIMING IF EVERYTHING ELSE IS WORKING CORRECT, RETARDING THE TIMING MAY MAKE IT PASS AND IT ALLSO CAN MAKE IT FAIL, RETARDING THE TIMING WILL CAUSE THE COMBUSTION TEMP TO GO UP ,THAT IS NOT WHAT YOU WANT TO CONTROL NOX,NOX IS PRIMARILY CAUSED BY COMBUSTION HEAT , FIRST GET THE IDOL STABLIZED , CHECK THE AIR PIPE GOING ACROSS THE VLV CVR FOR A HOLE BURNED IN THE BOTTOM SIDE ,IT WILL CAUSE THE IDOL TO BE ALL OVER THE PLACE , THEN EGR MUST BE WORKING CORRECT AND THE TPS ADJUSTMENT, IF ALL THIS IS WORKING , YOU PROB, HAVE A DEAD OR MARGINAL CAT, YOU CAN CHECK IT BUT FIRST MAKE SURE THE AIR INJ SYSTEM IS WORKING OK ALLSO, AFTER YOU GET IT SETTLED DOWN, GET YOURSELF AN INFARED THERMOMETER AND HOLD THE ENG SPEED AT 1500 RPM FOR ABOUT 2 MIN AS STEADY AS YOU CAN, KEEP IT AT THIS SPEED AND CHECK THE INLET TEMP OF THE CAT AND THE OUTLET TEMP ,THE OUTLET SHOULD BE AT LEAST 350-400 DEGREES HIGHER AT THIS POINT THAN THE INLET, IF IT IS VERY CLOSE INLET AND OUTLET REPLACE THE CONVERTOR , IT HAS THE OLD GM PANCAKE CONV, AND THEY ARE NOT KNOWN FOR LONG LIFE AFTER 125000 OR SO. , I RPLACE THEM ALL THE TIME WITH THE NEW HONEYCOMB THREE WAY CAT AND THEY NEVER FAIL TO PASS, WITH READING AS LOW AS ANY 2000 MODEL VEH. JERRY