Mondeo ST220 O2 sensors

[jungle.monkey]

Hi,
New here. Can anyone tell me how many O2 sensors are fitted to a 2002 3.0 Mondeo ST220? I assumed four, but only three are showing up on the scan - there's no O2S22...

[FORScan]

O2S11 - Bank 1, before the cat (cylinders 1-3)
O2S12 - Bank 2, before the cat (cylinders 4-6)
O2S21 - after the cat

[jungle.monkey]

Thanks