Replaced the water pump on my 94 SL 500. After the 4th or 5th start the CEL stayed on. Checked for codes and found code 4 on pin 4 which as near as I can determine is the AIR pump. A recheck seemed to confirm that as the vacuum hose to the pump was disconnected. I reconnected it but after 4 tries I have not been able to cancel the code.

Any ideas?