From a number of posts on the expansion tank it seems desirable that this float switch actually works.
If I take off the filler cap (engine cold of course) and push the float down with a bit of wire what will happen ? (sensibe answers only please).
Will the 'Low Coolant Level' warning reset automatically as soon as I release it or does it trigger a fault state that needs resetting ?
Does the engine need to be running or just ignition on ?

 

The switch will default to "low coolant" if it is broken. There is no need to check to see if it is working...if you don't have a "Low Coolant" message then you know 2 things (provided you haven't unplugged and jumpered the switch):

1. You have good coolant level
2. Sensor is functioning properly

when it malfunctions it will throw a false positive indicating "Low Coolant" but impossible for it to throw false negative

 

Thank you, that's all I need.