catalyst efficiency

Either your catalytic converter is gone already or the Up and Down stream oxgen sensor/s are not switching-get the fuel trimming check on the oxygen sensors.

Converters can also be damaged if they overheat due to ignition misfiring that allows unburned fuel to pass through into the exhaust (check for a fouled spark plug or bad plug wire). The same thing can happen if the engine has a bad exhaust valve that leaks compression into the exhaust (check