how long ago(miles) did they get changed?
how long were you driving around with these codes?
the reason your not getting good gas mileage is because with these codes present your car is running rich because its doesn't know what the sensors are reading it basically goes to a default setting,
and buy running rich all the time its dumping raw fuel in the exhaust which is very bad for the cat and if the damage is to late then the cat is toast and will have to be replaced but hopefully youll get lucky and yours will be ok
