1994 E420 code 4 and 5
 

My car has 233,000 miles and still runs well. It gets about 18mpg in mixed driving. Recently I discovered the check engine light was disabled. Once a new bulb was installed I found the CEL ON with blink codes 4 and 5. Resetting codes return after a week or so. My air pump has suction and vacuum lines are good. Should I install a new O2 sensor, as it has never been replaced? Are these codes somehow related? What senses EGR failure on these cars? Any assistance is appreciated!