2001 mitsubishi galant with a P0421 code whut to do

p0421 http://engine-codes.com/p0421_mitsubishi.html

p0171 http://engine-codes.com/p0171_mitsubishi.html

Solenoid is your problem, the solenoid is connected to the (EGR) exhaust gas resirculation, when the solenoid fail, the EGR don't work right, and your O2 sensor on the cat is reading to much exhaust gases in the cat, so your catalytic converter is failing, solution go to a junkyard and get one from other engine or similar the looks clean and replaced, run your car for more than 10 miles and your service soon engine light should go off, if that don't append your O2sensor on the cat is no good.

http://www.obd-codes.com/p0421

P0421....Warm Up Catalyst Efficiency Below Threshold (Bank 1)

the the heaters on the O2 sensors -two white wires on sensor check for continueity

I believe this has to do with your catalytic converter. Sounds like it might be getting old. The older they get the slower they are to respond. this is due to contamination. Same is true for your Oxygen sensors.

yes this is your catalytic convertor is clogged you have to change before is damages ur o2 sensors

The code means Warm-up catalyst efficiency below threshold.....

P0421 : Warm Up Catalyst Efficiency Below Threshold

DTC code P0421 is the O2 sensor that indicates a an issue with the Catalytic Converter. Replace the O2 sensor to the Catalytic Converter "the one after Converter" It's common for the sensor to fail and you can pick one up from Auto Zone and they will have the step by step instructions to replacing the O2 sensor. 
DTC code P0302 Indicates a misfire on cylinder #2. Address the spark plug, spark plug wire, and injector. Look for any damages and try running some fuel injector cleaner through the fuel system to clean out the injectors. 
Good luck and hope this helps. 

I have the same vehicle, same code P0421, changed the BOTH O2 sensors and the check engine light was still on, took it to my fathers garage and checked the Catalytic Converter, and it was ok later I found out that P0421 means that the vehicle doesn't have the proper mixture of gas and air (running richer) so I changed the MAX air flow sensor and wala the code went away.