Heated oxygen sensors (HO2S): Notes

5 cylinder engines have two heated oxygen sensors (HO2S), front and rear
6 cylinder engines have four heated oxygen sensors (HO2S), front and rear for bank 1 (cylinders 1-3) and front and rear for bank 2 (cylinders 4-6)
XC90s with 6 cylinder engines utilize the Y-concept, where the engine has two heated oxygen sensors (HO2S) bank 1 (cyl. 1 -3), bank 2 (cyl. 4 -6) and a rear heated oxygen sensor (HO2S) (bank 1).