These were always hard to test, they never really took off and didn't stay fitted for long.

I wouldn't get too hung up at looking for a reading, nitrogen oxides are formed at very high combustion temperatures, usually at high loads with levels of detonation. You're not going to really going to get any usable data in the workshop, a static voltage would probably be correct, they also have a specific type of cat fitted to this model too. I would be looking elsewhere for an emission fault.

Apologies in advance if I have this wrong, but it's always been my understanding.

What fault codes and emission data are you getting?