1998 Accord LX V6. Failing California smog on high NOx & high HC

Original Converter was stolen. Mechanic welded an aftermarket converter on but the car wasn't smogged until a year later (when it failed), so its not under warranty anymore

However, the computer is throwing NO codes. Can the catalytic converter be bad even without a P0420 code?
Additional Complication: Original computer failed last year & was replaced. Is it possible the new computer is bad & that's why its not throwing any codes?

Trying to avoid shelling out $600 for a MagnaFlow CARB converter + $150 to weld it on just for it to still fail smog afterwards.

Any ideas on how I should proceed?

 

California catalytic converters carry a 10 year minimum warranty.

 

Edmunds related site ideas below. Some causes listed may contribute to either failure and some may not throw a code.

possible causes of high HC
possible causes of high NO

FWIW, the first sentence of this paragaph from a Walker Exhaust site COULD be relevant but it would surprise me if it is the only cause of the out of spec measurements:

"... CARB-compliant catalytic converters typically use an enhanced combination of washcoat technology and higher precious metal load to meet the stricter emissions conversions standard. A CARB-compliant converter will also have an Executive Order (EO) number/stamp on its body. The stamped number on the converter body must match the correct EO number listed on the California Air Resources Board website for aftermarket catalytic converters."