Not sure what the set up is on the protege but for my 1998 Saturn the bank one 02 sensor is just below the outlet of the exhaust manifold. the car has 261000 miles on it so its not worth putting a pile of money in it . Heres what I did. Remove the 02 sensor, pour 4 to 6 ounces of seafoam down the hole ,reinstall the 02 sensor (remember to put antisieze on the threads) let it sit overnight. reset code and drive. It will put out a fair amount of white smoke, but that will clear after driving a few miles. after 1000 miles the p0420 has not returned. Will repeat process if it does.