[rodnic]

Not sure what is going on but here is the problem. My mileage has dropped about 4 mpg in the last month. No CEL but the "maintenance required" light blinks 8 times and then shuts off. I changed the oil 2,000 miles ago and reset the light as usual. I do have a sooty exhaust which I haven't had before. Air filter (KN) is clean. I know it sounds like a rich condition and I suspected an O2 sensor but wouldn't that throw a CEL? Car has 90K, has been trouble-free and actually runs very good. Any ideas? I am stumped! TIA