My 2010 G37 was throwing two codes saying that banks 1 & 2 were running lean and it was causing the car to idle at 1200 RPM's. I cleaned the throttle bodies, MAF sensors, and replaced the air filters. I reset the ECU but after a few days the problem kept coming back. I was smelling a slight coolant odor but couldn't find a leak. Then the connector between the heater hoses snapped and I replaced it with a metal connector and two new hoses. I don't understand why but this solved the problem with the engine running lean and idling high. It's been two weeks and the car runs great. Hopefully, this will apply to someone else's problem of a high idle and running lean.

 

Thanks for this post.....this happened to me too, idle didn't go that high but got the lean condition and the coolant hose/connector issues etc were all there and corrected by replacing the hoses and other stuff

....the problem is that he upstream O2 sensor is right below where that connector is between the heater hoses...when it leaks it leaks onto the O2 sensor...you're lucky it didnt ruin the sensor...in my case i had to change the sensor too, probably because I let it happen for a while

 

Subd for future issue.
thabks op. And hashim.