has anyone watched what reading you get from these sensors on a diagnostic machine?

the reason i ask is that i have been looking at mine today and the one in the pre cat jumps all over the place when on tick over and the one after the cat is nice and steady then when holding the engine at steady revs they both jumpa round all over the place like mad!

Also while holding steady revs the sensor in the pre cat reads on/off/on/off and so on all the time for the heated oxygen sensor while the one in the pre cat stays on off.

Any help would be great thanks

kev