Code 45 ?

What does a code 45 mean and what is the cure. I took off the egr valve and cleaned it . It had carbon holding it open and I got so it moves freely, it's on a 92 blazer with a 4.3 Vortec

02 sensor is picking up a rich mixture, many causes here, low vacuum to map sensor, engine misfire, high fuel pressure, fuel contaminated oil.