To be honest I have the 1652 and can't say that I have nulled them for live tests. I seem to remember the machine not liking it if i tried? Usually I null them ready for dead tests, the machine then recognises that they have been nulled so I doesn't matter if you switch across the different tests it remains nulled. Try it with continuity, null the leads to carry out various tests then check back on continuity should find it remains nulled. That's how I understand it to work?