catalytic converter gets red hot on 1997 Buick Park Avenue

i had valve seal problem and map sensor needed the old cat was red hot with flames when i removed cat insides were broken apart replaced cat replaced oil seals,map sensor,engine runs well idles well doesn't take the gas well i think there may be blockage but not sure what else could cause the cat to get red hot am getting po102 mass air flow sensor circuit-low frequency detected. thank you LC

Asked by for the 1997 Buick Park Avenue
You could have pieces of the cat plugging up the muffler causing a restriction in the exhaust.