These typically indicate that the heater element inside the O2 sensor(s) has burnt out.
Each O2 sensor has it's own little heating element to help bring the sensor up to operating temperature faster, for more reliable emissions readings when the engine is still warming up.
The Denso sensors used by Mitsubishi (and Toyota, and many other Japanese manufacturers) have a bad reputation for burning out the heater elements, and these codes almost always indicate just that.
Because all four sensors are powered by the MFI relay and only two sensors are failing, you can rule out that as the cause. The ground for the heaters is supplied by the PCM, and is very rarely a failure point.
Verify your electrical connections for the O2 sensors in question (bank 2 sensor 1 and bank 2 sensor 2). They are the two driver side O2 sensors. If they are secure and there is no obvious wiring damage, it is pretty safe to assume they are burned out. Any further diagnosis would require a professional scan tool and a multimeter.
Replacement of the O2 sensors would cost as follows:
Bank 2 Sensor 1: $328.80 parts $54 labor
Bank 2 Sensor 2: $328.80 parts $54 labor
The parts prices is full retail price from the dealership. Your price may be lower, that is the maximum full retail listing. I strongly urge you not to use aftermarket sensors to save money. They do not typically fair well on Mitsubishis.
Any questions just ask, if I have been helpful please remember to click Accept when you are satisfied.
Edited by Doug C on 12/2/2009 at 3:25 AM EST