Your code means the system is at its max adjustment and can no longer adapt to an air/fuel imbalance..
This can be caused by a lean condition, in which case the vac lines shoud all be checked for possible leaks [ on that chassis , check around the front of the engine, the line that go to the switch-over valves for EGR]

Also can be caused by a rich condition . In that case , a possible is the fuel regulator has a ruptured diaphragm . This can be checked by taking the hose off the reg. and looking for fuel in the hose.. there should be none.
A fuel pressure test is in order to complete this test.
And then , there is the possibility that the MAF is not reporting the correct intake air mass , in which case it is faulty [ a good possibity].. but , the proper sequence is start with the vav leak/fuel pressure first.
Adapatation specs are used for this diagnosis, but that requires equipment , so for the diyer , do the above sequence .
You may just have a bad fuel feed, so you will know with the new filter....
" 02 Sensor "is the proper terminology..