A catalyst efficiency fault code can only be set by one thing: a failing catalytic converter. There is no other possible cause for this particular code.
When a catalytic converter is operating, the chemical reaction taking place inside uses up virtually all of the oxygen that remains in the exhaust gas after combustion in the engine. An OBD-2 compliant emissions system such as on your vehicle has at least 2 oxygen sensors located in the exhaust stream: one before the cat, and one located after the Cat. The one that is downstream from the cat is there for the sole purpose of monitoring the oxygen content of the exhaust leaving the catalytic converter to make sure it has decrreased while passing through. If the signals from the pre-cat and post-cat O2 sensors match too closely, it indicates that the re is insufficient reaction taking place inside the converter and the ECM stores this code. This particular code cannot be set by a faulty oxygen sensor or any other cause; it is only stored when all oxygen sensors are functioning correctly. You need a new catalytic converter to repair this fault.
You will want to find out exactly WHY the cat has failed before having a new one installed to prevent a repeat failure; a converter should last the lifetime of the vehicle. What damages Cats is either a long term rich fuel mixture condition, poisoning from use of improper fuel or oil additives, or an ongoing engine misfire condition. These things result in unvburned fuel exiting the engine in the exhaust, which ends up in the CAT where it burns. The result is a dramatic temperature increase inside the CAT that melts the internal ceramic, rendering it inoperative.
If this vehicle is less than 6 years old, you may wish to contact the dealership for whatever make of vehicle you have. Catalytic converters are covered under mandatory federal emissions warranties that usually go much longer then the basic vehicle warranty (as much as 6 years on some models). If your vehicle's CAT is still under federal warranty, you can have it replaced for free at the dealer; all it takes is a phone call with the vehicle's VIN number and mileage to find out. Catalytic converters are expensive items; if you can get one for free that is much preferable to paying for one!
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