I have a Focus 1.6 auto which is putting up P1131 - Lack of HO2S-11 Switch, Sensor Indicates Lean A HEGO sensor.
My car only does this when left idling for a period of time. Idling does not sound great and it sounds like a popping type noise coming from around right hand side of engine. Engine does not stall.
I have a read other threads on here about air hose issues and possible PCV valve but mine slighlt different.
It does not effect car performance but is annoying when EML comes on periodically.