Nissan Repair: 2001 nissan sentra, mass airflow sensor, nissan sentra

Question
the car began to run very rough/would stall when initially started and put in gear for the first 5 minutes after sitting for several hours (especially first thing in the morning after sitting all night). the service engine light was on and original mechanic replaced an oxygen sensor based on code in computer. while driving the car home, the service engine light came back on. the original mechanic then did something else with the oxygen sensor based on computer code. again, when driving home, the service engine light came on again. gas gage didn't appear to be functioning properly. family friend seemed to think both problems might be related to the fuel filter/fuel pump. family friend replaced fuel pump. didn't fix the issue with car running rough/stalling and not sure about the gas gage. service engine light came on after a few days so took car back to original mechanic. original mechanic has indicated there are 4 or 5 codes in computer, but none of them seem to point in one direction as to what the problem might be. don't really want to begin replacing all of the sensors as this could be quite costly. any idea where to begin?

Answer
Susan,

It is a Sentra and the two most common things are the mass airflow sensor and the converter melting.  The mass airflow sensor will not throw a code so the mechanic will have to disconnect it and use a volt ohm meter to test it.  The converter will have to be removed if your mechanic does not know how to test it in the car.  Signs point to both of these.  About $300 for the mass airflow sensor and a little less for the converter.  Make sure the mechanic replaces any missing bolts on the converter if he replaces it or not.  Sentra cars are known for bolts coming out of the converter too.