P0421 Mazda - What systematic diagnostic tests or procedures would best confirm the root cause?

Question

I just corrected a P0302 code (Engine Misfire detected) by replacing all spark plugs and wire/boot sets. Engine idle was smooth and very quiet by comparison, engine power under load and responsiveness was improved, and "carbon" laced water no longer dripped from exhaust pipe. Other ancillary procedures were also done as maintenance: Engine oil was changed, battery terminal connections were cleaned and then it was tested (multimeter on battery terminals showed 12.4 v dropping to about 9 volts during start-up and engine turn-over). Also EGR valve was checked, along with intake hoses and air filter was replaced. After 2 days driving, the P0402 comes up.

Answer 1

P0402 EGR (exhaust gas recirculation) excessive flow detected

P0421 Warm up catalytst system efficiency below threshold

Excessive flow on the EGR ,the vacuum hose routing need to be checked for this condition , this excesive EGR flow can also cause P0421. As for the battery test 12.4 volts , the battery condition is considered low, the minimun  is 12.5. as for starting the minimum cranking  voltage is 9.5 volts.