Trouble code 11 on W124
 

Every 20 or so start cycles I get a check engine light with a code 11 on my 1990 W124 300E 3.0L. From what I can find it's related to the air injection system, but I would like to verify that first. When it first happened 4 years ago, I exchanged the air pump for a rebuilt one. Now it's happening again but the pump seems to be working. Is there another component of the system that fails or that might be a likely culprit.