I recently took my 2000 E430 4matic in to the dealer to address the CEL. He replaced the left rear o2 sensor and this seemingly solved the problem. Now 500 miles later the cel has come on again. I am wondering if the MAF could have thrown the o2 sensor fault in the first place and maybe that should have been replaced instead of the sensor. I am planning to call the dealer tomorrow but was looking for info before I do. Thanks in advance.