I am assuming that the P0241 should read P0421. There is no P0241 for your car and the P0421 fits the situation you are describing.
The P0421 code is set if the quotent of the frequency of the rear O2 sensor divided by the frequency of the front O2 sensor is above 0.8. This indicates that the front sensor is not active or the two sensors are tracking in parallel. The two sensors monitor the upstream catalytic converter so this code does not relate to the down stream catalytic converter. The failure of the down stream catalytic converter can only be checked by doing an tail pipe exhaust measurement.
A Mitsubishi service department should have a scanner that can monitor the activity of the rear and front O2 sensors. If the rear O2 sensor were not active, IIRC another P code would be set. If the front O2 sensor is not active, its replacement might solve your problem. If the scanner indicates that the front and rear sensor are tracking together, it would appear that the exhaust manifold up stream catalytic converter has lost active surface and must be replaced to pass DEQ.
Hope this helps.