What is the function of the EGR in a 3rd gen? It appears under diagnostics but Mazda mechanic says not really necessary to replace.

 

EGR stands for Exhaust Gas Recirculation valve. Some of the exhaust gas is diverted back into the combustion chamber under low load conditions. It's there to reduce NOx levels, but in a car that already runs very rich, it's totally useless. The '89-'91 2nd gens didn't have one at all, and only the California 3rd gens had it. It's just another useless piece of emissions equipment to clutter the engine bay.

 

Not necessary, as long as you change ECUs or sneak in a resistor to prevent the ECU from noticing.