Cadillac Repair: 1990 cadillac deville, cadillac deville, oxygen sensor

Question
I have a 90 cadillac deville with 114000 miles in excellent shape. Car runs rough misses at an idle and running speed. Gas mileage has dropped by about 5 miles/gallon. Computer codes are E13 and E49. Replaced all fuel injectors at 70000 miles and EGR valve. Recently replaced (10000 miles ago) plugs and distributor cap an rotor along with other regular maintenance. Smells of rich fumes at an idle. Could it be the injectors again what is a easy way to check them?

Answer
Replace the oxygen sensor and clear the code E13 from the computer.

E49 is an air system fault.

Replace the oxygen sensor, clean the spak plugs if fouled and CLEAR the codes from the computer and see which codes re-appear.

NOTE: Disconnecting the battery will NOT clear the codes.