Nissan Repair: catalytic converter system -efficiencybelow threshold(bank 1), threshold bank, mazda protege

Question
what causes this 1999 mazda protege 1.8 motor

Answer
Stanley,

Usually an over rich condition causes the problem.  Too much fuel equals a converter that cannot work properly.  I would check the fuel filter and replace it if you have not done so in the last couple of years.  Also, check the fuel pressure and make sure the plugs are recent too.  Defective plugs will not burn the fuel properly.  If all of these are good then the oxygen sensor itself needs to be replaced.  Now, if you have had an over rich condition for several months you could have damaged the converter.  That would mean a replacement of the converter.