The passages for the Exhaust Gas Recirculation (EGR) system can become restricted or clogged causing the Check Engine Light to illuminate. The EGR valve will need to be inspected and the passages cleaned.Google+
Car Problem Reports
Mazda Miata MX-5 Check Engine Light Due to Restricted EGR System
Mazda Miata MX-5 Problem
Average Mileage: 85,533 mi (54,600 mi - 110,000 mi)
Visitor, 2000 Mazda Miata MX-5, 54,600 mi
Engine light comes on when checked I get code 420 which has something to do with the o2 sensor and the catalac converter, both have been changed and the light still comes on.
Visitor, 2000 Mazda Miata MX-5, 110,000 mi
EGR valve failed & clogged. EGR replaced but problem quickly resurfaced. Turned out that the passageway in the valve cover was clogged, too, and had to be reamed out. That fixed the problem entirely. Runs great now.
Visitor, 1999 Mazda Miata MX-5, 92,000 mi
The check engine light came on and the code read for the EGR system. I replaced egr valve and the light didn't turn off. I had to clean out the throttle body in order to get the light to turn off.