Timbo

Know your PO2 simply means that if the setpoint on your unit is say 1.3, then look at the displays and make sure you see 1.3 or thereabouts on all 3 sensors. So if u look and see something different a cell may be wrong or solenoid is stuck open/closed or other problems.
So a good way to verify which cells are okay is below.

It is useful to have on a slate on your wrist what PO2 you would expect to see if you did a diluent flush at 10M increments. eg

On air 21% O2

10M = 2Bar so dil flush is 0.21X2=0.42
20M = 3Bar so dil flush is 0.21X3=0.63
30M = 4Bar so dil flush is 0.21X4=0.84
40M = 5Bar so dil flush is 0.21X5=1.05

So on your slate just write
10M=0.42
20M=0.63 and so on.

So if you want to verify cells do a diluent flush and say you were at 38M then expect to see the cells crash down to around 1.05.
If they are near to this all is okay.
If 2 read around 1.05 and say one read 0.63, then that cell has probably had it.
Always know that you PO2 is what you want, if it is wrong do what you were trained to do on the course to rectify it.
In early days, if in doubt BAIL OUT TO OPEN CIRCUIT.
Hope this helps.
Tim B