The age of the O2 sensor upstream is unknown (same with downstream). It's worth replacing it as the car has 129,000 miles on it. As for the P0243, we have confirmed there are no cracks in vacuum lines or the charge pipes (input and output intercooler pipes) and hoses to the MAF. I guess tomorrow I'll find out what the diagnostic says about the ECU and the emissions monitors. Again, this is not and SDS diagnostic from Mercedes.