To clarify the state of the cells in my machine.
All of them work, reading low and high. All of them show 1.6 at 6m following an oxygen flush.
One cell, which I'll call Paulo, is a bit slower than the rest and this has been moved off of the controller and on to the backup. It eventually reads OK but has a slightly lower mV reading and seems to need to warm up. This sensor will be the first to be replaced once cells are available.
All others are well within tolerance and read the same.