e46 O2 Sensor bank 1 & 2 - control limit

Car felt a bit 'slow' this week so did another scan to check what might pop up. Recently replaced the MAF sensor. Still need to replace the crankshaft sensor. Have the part but can't attend to it now.

The scan showed that the Oxygen sensors on Bank 1 (Cyl 1 -3) and bank 2 (Cyl 4 - 6) are throwing a 'control limit' signal.

I did some research and it seems that it is unlikely for both Oxygen sensors to go at the same time. What most suggest is that the error report means there is a air leak in the system somewhere. The O2 sensors cannot tell the ECU what the fuel/air mixture should be as the air entiring casnnot be measured within the parameters.

Thus the car might be under fuelling (running lean) or over fuelling. Car is running heavy on petrol at the moment. I cannot find any obvious leaks from the air intake, MAF sensor, Upper intake or lower intake boot.

Any advise on what else it might be will be greatly appreciated.

Thanks!