recently i replaced my front and back oxygen sensors as well as my egr valve after my car failed an emissions test and gave those codes i retested and i still failed with the egr and rear oxygen sensor still sending a code the only new code is po100 which says mass or volume air flow A circuit the only other repair recently made was i replaced the egr tube because the old one had been cut by a previous shop and when they put a hose over the open space it burned up and i was not able to find anything to breach that gap so i finally found a car that someone was scrapingand pulled that part.
RepairPal Certified shops can diagnose and repair your 1998 Nissan Sentra. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
You have to be sure that the EGR passages in the intake manifold are clear and the EGR is getting the proper vacuum signal when it should. The rear O2 sensor reading wrong could be because your catalytic converter is failing, but that's just a guess.
The airflow sensor code is either a circuit problem or that the sensor is not providing believable input to the controller. Check that the sensor is mounted correctly and there are no air leaks at loose clamps or defective ducting to the intake manifold.
Here's a directory link for you:
http://repairpal.com/nissan-repair-in-churchville-maryland
